// Written in the D programming language.

/**
This module implements a variety of type constructors, i.e., templates
that allow construction of new, useful general-purpose types.

$(SCRIPT inhibitQuickIndex = 1;)
$(DIVC quickindex,
$(BOOKTABLE,
$(TR $(TH Category) $(TH Symbols))
$(TR $(TD Tuple) $(TD
    $(LREF isTuple)
    $(LREF Tuple)
    $(LREF tuple)
    $(LREF reverse)
))
$(TR $(TD Flags) $(TD
    $(LREF BitFlags)
    $(LREF isBitFlagEnum)
    $(LREF Flag)
    $(LREF No)
    $(LREF Yes)
))
$(TR $(TD Memory allocation) $(TD
    $(LREF SafeRefCounted)
    $(LREF safeRefCounted)
    $(LREF RefCountedAutoInitialize)
    $(LREF scoped)
    $(LREF Unique)
))
$(TR $(TD Code generation) $(TD
    $(LREF AutoImplement)
    $(LREF BlackHole)
    $(LREF generateAssertTrap)
    $(LREF generateEmptyFunction)
    $(LREF WhiteHole)
))
$(TR $(TD Nullable) $(TD
    $(LREF Nullable)
    $(LREF nullable)
    $(LREF NullableRef)
    $(LREF nullableRef)
))
$(TR $(TD Proxies) $(TD
    $(LREF Proxy)
    $(LREF rebindable)
    $(LREF Rebindable)
    $(LREF ReplaceType)
    $(LREF unwrap)
    $(LREF wrap)
))
$(TR $(TD Types) $(TD
    $(LREF alignForSize)
    $(LREF Ternary)
    $(LREF Typedef)
    $(LREF TypedefType)
    $(LREF UnqualRef)
))
))

Copyright: Copyright the respective authors, 2008-
License:   $(HTTP boost.org/LICENSE_1_0.txt, Boost License 1.0).
Source:    $(PHOBOSSRC std/typecons.d)
Authors:   $(HTTP erdani.org, Andrei Alexandrescu),
           $(HTTP bartoszmilewski.wordpress.com, Bartosz Milewski),
           Don Clugston,
           Shin Fujishiro,
           Kenji Hara
 */
module std.typecons;

import std.format.spec : singleSpec, FormatSpec;
import std.format.write : formatValue;
import std.meta : AliasSeq, allSatisfy;
import std.range.primitives : isOutputRange;
import std.traits;
import std.internal.attributes : betterC;

/// Value tuples
@safe unittest
{
    alias Coord = Tuple!(int, "x", int, "y", int, "z");
    Coord c;
    c[1] = 1;       // access by index
    c.z = 1;        // access by given name
    assert(c == Coord(0, 1, 1));

    // names can be omitted, types can be mixed
    alias DictEntry = Tuple!(string, int);
    auto dict = DictEntry("seven", 7);

    // element types can be inferred
    assert(tuple(2, 3, 4)[1] == 3);
    // type inference works with names too
    auto tup = tuple!("x", "y", "z")(2, 3, 4);
    assert(tup.y == 3);
}

/// Rebindable references to const and immutable objects
@safe unittest
{
    class Widget
    {
        void foo() const @safe {}
    }
    const w1 = new Widget, w2 = new Widget;
    w1.foo();
    // w1 = w2 would not work; can't rebind const object

    auto r = Rebindable!(const Widget)(w1);
    // invoke method as if r were a Widget object
    r.foo();
    // rebind r to refer to another object
    r = w2;
}

/**
Encapsulates unique ownership of a resource.

When a `Unique!T` goes out of scope it will call `destroy`
on the resource `T` that it manages, unless it is transferred.
One important consequence of `destroy` is that it will call the
destructor of the resource `T`.  GC-managed references are not
guaranteed to be valid during a destructor call, but other members of
`T`, such as file handles or pointers to `malloc` memory, will
still be valid during the destructor call.  This allows the resource
`T` to deallocate or clean up any non-GC resources.

If it is desirable to persist a `Unique!T` outside of its original
scope, then it can be transferred.  The transfer can be explicit, by
calling `release`, or implicit, when returning Unique from a
function. The resource `T` can be a polymorphic class object or
instance of an interface, in which case Unique behaves polymorphically
too.

If `T` is a value type, then `Unique!T` will be implemented
as a reference to a `T`.
*/
struct Unique(T)
{
/** Represents a reference to `T`. Resolves to `T*` if `T` is a value type. */
static if (is(T == class) || is(T == interface))
    alias RefT = T;
else
    alias RefT = T*;

public:
    // Deferred in case we get some language support for checking uniqueness.
    version (None)
    /**
    Allows safe construction of `Unique`. It creates the resource and
    guarantees unique ownership of it (unless `T` publishes aliases of
    `this`).
    Note: Nested structs/classes cannot be created.
    Params:
    args = Arguments to pass to `T`'s constructor.
    ---
    static class C {}
    auto u = Unique!(C).create();
    ---
    */
    static Unique!T create(A...)(auto ref A args)
    if (__traits(compiles, new T(args)))
    {
        Unique!T u;
        u._p = new T(args);
        return u;
    }

    /**
    Constructor that takes an rvalue.
    It will ensure uniqueness, as long as the rvalue
    isn't just a view on an lvalue (e.g., a cast).
    Typical usage:
    ----
    Unique!Foo f = new Foo;
    ----
    */
    this(RefT p)
    {
        _p = p;
    }
    /**
    Constructor that takes an lvalue. It nulls its source.
    The nulling will ensure uniqueness as long as there
    are no previous aliases to the source.
    */
    this(ref RefT p)
    {
        _p = p;
        p = null;
        assert(p is null);
    }
    /**
    Constructor that takes a `Unique` of a type that is convertible to our type.

    Typically used to transfer a `Unique` rvalue of derived type to
    a `Unique` of base type.
    Example:
    ---
    class C : Object {}

    Unique!C uc = new C;
    Unique!Object uo = uc.release;
    ---
    */
    this(U)(Unique!U u)
    if (is(u.RefT:RefT))
    {
        _p = u._p;
        u._p = null;
    }

    /// Transfer ownership from a `Unique` of a type that is convertible to our type.
    void opAssign(U)(Unique!U u)
    if (is(u.RefT:RefT))
    {
        // first delete any resource we own
        destroy(this);
        _p = u._p;
        u._p = null;
    }

    ~this()
    {
        if (_p !is null)
        {
            static if (is(T == class) || is(T == interface))
                destroy(_p);
            else
                destroy(*_p);
            _p = null;
        }
    }

    /** Returns whether the resource exists. */
    @property bool isEmpty() const
    {
        return _p is null;
    }
    /** Transfer ownership to a `Unique` rvalue. Nullifies the current contents.
    Same as calling std.algorithm.move on it.
    */
    Unique release()
    {
        import std.algorithm.mutation : move;
        return this.move;
    }

    /** Forwards member access to contents. */
    mixin Proxy!_p;

    /**
    Postblit operator is undefined to prevent the cloning of `Unique` objects.
    */
    @disable this(this);

private:
    RefT _p;
}

///
@safe unittest
{
    struct S
    {
        int i;
        this(int i){this.i = i;}
    }
    Unique!S produce()
    {
        // Construct a unique instance of S on the heap
        Unique!S ut = new S(5);
        // Implicit transfer of ownership
        return ut;
    }
    // Borrow a unique resource by ref
    void increment(ref Unique!S ur)
    {
        ur.i++;
    }
    void consume(Unique!S u2)
    {
        assert(u2.i == 6);
        // Resource automatically deleted here
    }
    Unique!S u1;
    assert(u1.isEmpty);
    u1 = produce();
    assert(u1.i == 5);
    increment(u1);
    assert(u1.i == 6);
    //consume(u1); // Error: u1 is not copyable
    // Transfer ownership of the resource
    consume(u1.release);
    assert(u1.isEmpty);
}

@safe unittest
{
    int i;
    struct S
    {
        ~this()
        {
            // check context pointer still exists - dtor also called before GC frees struct
            if (this.tupleof[0])
                i++;
        }
    }
    {
        Unique!S u = new S;
    }
    assert(i == 1);
}

@system unittest
{
    // test conversion to base ref
    int deleted = 0;
    class C
    {
        ~this(){deleted++;}
    }
    // constructor conversion
    Unique!Object u = Unique!C(new C);
    static assert(!__traits(compiles, {u = new C;}));
    assert(!u.isEmpty);
    destroy(u);
    assert(deleted == 1);

    Unique!C uc = new C;
    static assert(!__traits(compiles, {Unique!Object uo = uc;}));
    Unique!Object uo = new C;
    // opAssign conversion, deleting uo resource first
    uo = uc.release;
    assert(uc.isEmpty);
    assert(!uo.isEmpty);
    assert(deleted == 2);
}

@system unittest
{
    class Bar
    {
        ~this() { debug(Unique) writeln("    Bar destructor"); }
        int val() const { return 4; }
    }
    alias UBar = Unique!(Bar);
    UBar g(UBar u)
    {
        debug(Unique) writeln("inside g");
        return u.release;
    }
    auto ub = UBar(new Bar);
    assert(!ub.isEmpty);
    assert(ub.val == 4);
    static assert(!__traits(compiles, {auto ub3 = g(ub);}));
    auto ub2 = g(ub.release);
    assert(ub.isEmpty);
    assert(!ub2.isEmpty);
}

@system unittest
{
    interface Bar
    {
        int val() const;
    }
    class BarImpl : Bar
    {
        static int count;
        this()
        {
            count++;
        }
        ~this()
        {
            count--;
        }
        int val() const { return 4; }
    }
    alias UBar = Unique!Bar;
    UBar g(UBar u)
    {
        debug(Unique) writeln("inside g");
        return u.release;
    }
    void consume(UBar u)
    {
        assert(u.val() == 4);
        // Resource automatically deleted here
    }
    auto ub = UBar(new BarImpl);
    assert(BarImpl.count == 1);
    assert(!ub.isEmpty);
    assert(ub.val == 4);
    static assert(!__traits(compiles, {auto ub3 = g(ub);}));
    auto ub2 = g(ub.release);
    assert(ub.isEmpty);
    assert(!ub2.isEmpty);
    consume(ub2.release);
    assert(BarImpl.count == 0);
}

@safe unittest
{
    struct Foo
    {
        ~this() { }
        int val() const { return 3; }
        @disable this(this);
    }
    alias UFoo = Unique!(Foo);

    UFoo f(UFoo u)
    {
        return u.release;
    }

    auto uf = UFoo(new Foo);
    assert(!uf.isEmpty);
    assert(uf.val == 3);
    static assert(!__traits(compiles, {auto uf3 = f(uf);}));
    auto uf2 = f(uf.release);
    assert(uf.isEmpty);
    assert(!uf2.isEmpty);
}

// ensure Unique behaves correctly through const access paths
@system unittest
{
    struct Bar {int val;}
    struct Foo
    {
        Unique!Bar bar = new Bar;
    }

    Foo foo;
    foo.bar.val = 6;
    const Foo* ptr = &foo;
    static assert(is(typeof(ptr) == const(Foo*)));
    static assert(is(typeof(ptr.bar) == const(Unique!Bar)));
    static assert(is(typeof(ptr.bar.val) == const(int)));
    assert(ptr.bar.val == 6);
    foo.bar.val = 7;
    assert(ptr.bar.val == 7);
}

// Used in Tuple.toString
private template sharedToString(alias field)
if (is(typeof(field) == shared))
{
    static immutable sharedToString = typeof(field).stringof;
}

private template sharedToString(alias field)
if (!is(typeof(field) == shared))
{
    alias sharedToString = field;
}

private enum bool distinctFieldNames(names...) = __traits(compiles,
{
    static foreach (__name; names)
        static if (is(typeof(__name) : string))
            mixin("enum int " ~ __name ~ " = 0;");
});

@safe unittest
{
    static assert(!distinctFieldNames!(string, "abc", string, "abc"));
    static assert(distinctFieldNames!(string, "abc", int, "abd"));
    static assert(!distinctFieldNames!(int, "abc", string, "abd", int, "abc"));
    // https://issues.dlang.org/show_bug.cgi?id=19240
    static assert(!distinctFieldNames!(int, "int"));
}


// Parse (type,name) pairs (FieldSpecs) out of the specified
// arguments. Some fields would have name, others not.
private template parseSpecs(Specs...)
{
    static if (Specs.length == 0)
    {
        alias parseSpecs = AliasSeq!();
    }
    else static if (is(Specs[0]))
    {
        static if (is(typeof(Specs[1]) : string))
        {
            alias parseSpecs =
                AliasSeq!(FieldSpec!(Specs[0 .. 2]),
                          parseSpecs!(Specs[2 .. $]));
        }
        else
        {
            alias parseSpecs =
                AliasSeq!(FieldSpec!(Specs[0]),
                          parseSpecs!(Specs[1 .. $]));
        }
    }
    else
    {
        static assert(0, "Attempted to instantiate Tuple with an "
                        ~"invalid argument: "~ Specs[0].stringof);
    }
}

private template FieldSpec(T, string s = "")
{
    alias Type = T;
    alias name = s;
}

// Used with staticMap.
private alias extractType(alias spec) = spec.Type;
private alias extractName(alias spec) = spec.name;
private template expandSpec(alias spec)
{
    static if (spec.name.length == 0)
        alias expandSpec = AliasSeq!(spec.Type);
    else
        alias expandSpec = AliasSeq!(spec.Type, spec.name);
}


private enum areCompatibleTuples(Tup1, Tup2, string op) =
    isTuple!(OriginalType!Tup2) && Tup1.Types.length == Tup2.Types.length && is(typeof(
    (ref Tup1 tup1, ref Tup2 tup2)
    {
        static foreach (i; 0 .. Tup1.Types.length)
        {{
            auto lhs = typeof(tup1.field[i]).init;
            auto rhs = typeof(tup2.field[i]).init;
            static if (op == "=")
                lhs = rhs;
            else
                auto result = mixin("lhs "~op~" rhs");
        }}
    }));

private enum areBuildCompatibleTuples(Tup1, Tup2) =
    isTuple!Tup2 && Tup1.Types.length == Tup2.Types.length && is(typeof(
    {
        static foreach (i; 0 .. Tup1.Types.length)
            static assert(isBuildable!(Tup1.Types[i], Tup2.Types[i]));
    }));

// Returns `true` iff a `T` can be initialized from a `U`.
private enum isBuildable(T, U) = is(typeof(
    {
        U u = U.init;
        T t = u;
    }));
// Helper for partial instantiation
private template isBuildableFrom(U)
{
    enum isBuildableFrom(T) = isBuildable!(T, U);
}

private enum hasCopyCtor(T) = __traits(hasCopyConstructor, T);

// T is expected to be an instantiation of Tuple.
private template noMemberHasCopyCtor(T)
{
    import std.meta : anySatisfy;
    enum noMemberHasCopyCtor = !anySatisfy!(hasCopyCtor, T.Types);
}

/**
_Tuple of values, for example $(D Tuple!(int, string)) is a record that
stores an `int` and a `string`. `Tuple` can be used to bundle
values together, notably when returning multiple values from a
function. If `obj` is a `Tuple`, the individual members are
accessible with the syntax `obj[0]` for the first field, `obj[1]`
for the second, and so on.

See_Also: $(LREF tuple).

Params:
    Specs = A list of types (and optionally, member names) that the `Tuple` contains.
*/
template Tuple(Specs...)
if (distinctFieldNames!(Specs))
{
    import std.meta : staticMap;

    alias fieldSpecs = parseSpecs!Specs;

    // Generates named fields as follows:
    //    alias name_0 = Identity!(field[0]);
    //    alias name_1 = Identity!(field[1]);
    //      :
    // NOTE: field[k] is an expression (which yields a symbol of a
    //       variable) and can't be aliased directly.
    enum injectNamedFields = ()
    {
        string decl = "";
        static foreach (i, val; fieldSpecs)
        {{
            immutable si = i.stringof;
            decl ~= "alias _" ~ si ~ " = Identity!(field[" ~ si ~ "]);";
            if (val.name.length != 0)
            {
                decl ~= "alias " ~ val.name ~ " = _" ~ si ~ ";";
            }
        }}
        return decl;
    };

    // Returns Specs for a subtuple this[from .. to] preserving field
    // names if any.
    alias sliceSpecs(size_t from, size_t to) =
        staticMap!(expandSpec, fieldSpecs[from .. to]);

    struct Tuple
    {
        /**
         * The types of the `Tuple`'s components.
         */
        alias Types = staticMap!(extractType, fieldSpecs);

        private alias _Fields = Specs;

        ///
        static if (Specs.length == 0) @safe unittest
        {
            import std.meta : AliasSeq;
            alias Fields = Tuple!(int, "id", string, float);
            static assert(is(Fields.Types == AliasSeq!(int, string, float)));
        }

        /**
         * The names of the `Tuple`'s components. Unnamed fields have empty names.
         */
        alias fieldNames = staticMap!(extractName, fieldSpecs);

        ///
        static if (Specs.length == 0) @safe unittest
        {
            import std.meta : AliasSeq;
            alias Fields = Tuple!(int, "id", string, float);
            static assert(Fields.fieldNames == AliasSeq!("id", "", ""));
        }

        /**
         * Use `t.expand` for a `Tuple` `t` to expand it into its
         * components. The result of `expand` acts as if the `Tuple`'s components
         * were listed as a list of values. (Ordinarily, a `Tuple` acts as a
         * single value.)
         */
        Types expand;
        mixin(injectNamedFields());

        ///
        static if (Specs.length == 0) @safe unittest
        {
            auto t1 = tuple(1, " hello ", 'a');
            assert(t1.toString() == `Tuple!(int, string, char)(1, " hello ", 'a')`);

            void takeSeveralTypes(int n, string s, bool b)
            {
                assert(n == 4 && s == "test" && b == false);
            }

            auto t2 = tuple(4, "test", false);
            //t.expand acting as a list of values
            takeSeveralTypes(t2.expand);
        }

        static if (is(Specs))
        {
            // This is mostly to make t[n] work.
            alias expand this;
        }
        else
        {
            @property
            ref inout(Tuple!Types) _Tuple_super() inout @trusted
            {
                static foreach (i; 0 .. Types.length)   // Rely on the field layout
                {
                    static assert(typeof(return).init.tupleof[i].offsetof ==
                                                       expand[i].offsetof);
                }
                return *cast(typeof(return)*) &(field[0]);
            }
            // This is mostly to make t[n] work.
            alias _Tuple_super this;
        }

        // backwards compatibility
        alias field = expand;

        /**
         * Constructor taking one value for each field.
         *
         * Params:
         *     values = A list of values that are either the same
         *              types as those given by the `Types` field
         *              of this `Tuple`, or can implicitly convert
         *              to those types. They must be in the same
         *              order as they appear in `Types`.
         */
        static if (Types.length > 0)
        {
            this(Types values)
            {
                field[] = values[];
            }
        }

        ///
        static if (Specs.length == 0) @safe unittest
        {
            alias ISD = Tuple!(int, string, double);
            auto tup = ISD(1, "test", 3.2);
            assert(tup.toString() == `Tuple!(int, string, double)(1, "test", 3.2)`);
        }

        /**
         * Constructor taking a compatible array.
         *
         * Params:
         *     values = A compatible static array to build the `Tuple` from.
         *              Array slices are not supported.
         */
        this(U, size_t n)(U[n] values)
        if (n == Types.length && allSatisfy!(isBuildableFrom!U, Types))
        {
            static foreach (i; 0 .. Types.length)
            {
                field[i] = values[i];
            }
        }

        ///
        static if (Specs.length == 0) @safe unittest
        {
            int[2] ints;
            Tuple!(int, int) t = ints;
        }

        /**
         * Constructor taking a compatible `Tuple`. Two `Tuple`s are compatible
         * $(B iff) they are both of the same length, and, for each type `T` on the
         * left-hand side, the corresponding type `U` on the right-hand side can
         * implicitly convert to `T`.
         *
         * Params:
         *     another = A compatible `Tuple` to build from. Its type must be
         *               compatible with the target `Tuple`'s type.
         */
        this(U)(U another)
        if (areBuildCompatibleTuples!(typeof(this), U) &&
            (noMemberHasCopyCtor!(typeof(this)) || !is(Unqual!U == Unqual!(typeof(this)))))
        {
            field[] = another.field[];
        }

        ///
        static if (Specs.length == 0) @safe unittest
        {
            alias IntVec = Tuple!(int, int, int);
            alias DubVec = Tuple!(double, double, double);

            IntVec iv = tuple(1, 1, 1);

            //Ok, int can implicitly convert to double
            DubVec dv = iv;
            //Error: double cannot implicitly convert to int
            //IntVec iv2 = dv;
        }

        /**
         * Comparison for equality. Two `Tuple`s are considered equal
         * $(B iff) they fulfill the following criteria:
         *
         * $(UL
         *   $(LI Each `Tuple` is the same length.)
         *   $(LI For each type `T` on the left-hand side and each type
         *        `U` on the right-hand side, values of type `T` can be
         *        compared with values of type `U`.)
         *   $(LI For each value `v1` on the left-hand side and each value
         *        `v2` on the right-hand side, the expression `v1 == v2` is
         *        true.))
         *
         * Params:
         *     rhs = The `Tuple` to compare against. It must meeting the criteria
         *           for comparison between `Tuple`s.
         *
         * Returns:
         *     true if both `Tuple`s are equal, otherwise false.
         */
        bool opEquals(R)(R rhs)
        if (areCompatibleTuples!(typeof(this), R, "=="))
        {
            return field[] == rhs.field[];
        }

        /// ditto
        bool opEquals(R)(R rhs) const
        if (areCompatibleTuples!(typeof(this), R, "=="))
        {
            return field[] == rhs.field[];
        }

        /// ditto
        bool opEquals(R...)(auto ref R rhs)
        if (R.length > 1 && areCompatibleTuples!(typeof(this), Tuple!R, "=="))
        {
            static foreach (i; 0 .. Types.length)
                if (field[i] != rhs[i])
                    return false;

            return true;
        }

        ///
        static if (Specs.length == 0) @safe unittest
        {
            Tuple!(int, string) t1 = tuple(1, "test");
            Tuple!(double, string) t2 =  tuple(1.0, "test");
            //Ok, int can be compared with double and
            //both have a value of 1
            assert(t1 == t2);
        }

        /**
         * Comparison for ordering.
         *
         * Params:
         *     rhs = The `Tuple` to compare against. It must meet the criteria
         *           for comparison between `Tuple`s.
         *
         * Returns:
         * For any values `v1` contained by the left-hand side tuple and any
         * values `v2` contained by the right-hand side:
         *
         * 0 if `v1 == v2` for all members or the following value for the
         * first position were the mentioned criteria is not satisfied:
         *
         * $(UL
         *   $(LI NaN, in case one of the operands is a NaN.)
         *   $(LI A negative number if the expression `v1 < v2` is true.)
         *   $(LI A positive number if the expression `v1 > v2` is true.))
         */
        auto opCmp(R)(R rhs)
        if (areCompatibleTuples!(typeof(this), R, "<"))
        {
            static foreach (i; 0 .. Types.length)
            {
                if (field[i] != rhs.field[i])
                {
                    import std.math.traits : isNaN;
                    static if (isFloatingPoint!(Types[i]))
                    {
                        if (isNaN(field[i]))
                            return float.nan;
                    }
                    static if (isFloatingPoint!(typeof(rhs.field[i])))
                    {
                        if (isNaN(rhs.field[i]))
                            return float.nan;
                    }
                    static if (is(typeof(field[i].opCmp(rhs.field[i]))) &&
                               isFloatingPoint!(typeof(field[i].opCmp(rhs.field[i]))))
                    {
                        if (isNaN(field[i].opCmp(rhs.field[i])))
                            return float.nan;
                    }

                    return field[i] < rhs.field[i] ? -1 : 1;
                }
            }
            return 0;
        }

        /// ditto
        auto opCmp(R)(R rhs) const
        if (areCompatibleTuples!(typeof(this), R, "<"))
        {
            static foreach (i; 0 .. Types.length)
            {
                if (field[i] != rhs.field[i])
                {
                    import std.math.traits : isNaN;
                    static if (isFloatingPoint!(Types[i]))
                    {
                        if (isNaN(field[i]))
                            return float.nan;
                    }
                    static if (isFloatingPoint!(typeof(rhs.field[i])))
                    {
                        if (isNaN(rhs.field[i]))
                            return float.nan;
                    }
                    static if (is(typeof(field[i].opCmp(rhs.field[i]))) &&
                               isFloatingPoint!(typeof(field[i].opCmp(rhs.field[i]))))
                    {
                        if (isNaN(field[i].opCmp(rhs.field[i])))
                            return float.nan;
                    }

                    return field[i] < rhs.field[i] ? -1 : 1;
                }
            }
            return 0;
        }

        /**
            The first `v1` for which `v1 > v2` is true determines
            the result. This could lead to unexpected behaviour.
         */
        static if (Specs.length == 0) @safe unittest
        {
            auto tup1 = tuple(1, 1, 1);
            auto tup2 = tuple(1, 100, 100);
            assert(tup1 < tup2);

            //Only the first result matters for comparison
            tup1[0] = 2;
            assert(tup1 > tup2);
        }

        /**
         Concatenate Tuples.
         Tuple concatenation is only allowed if all named fields are distinct (no named field of this tuple occurs in `t`
         and no named field of `t` occurs in this tuple).

         Params:
             t = The `Tuple` to concatenate with

         Returns: A concatenation of this tuple and `t`
         */
        auto opBinary(string op, T)(auto ref T t)
        if (op == "~" && !(is(T : U[], U) && isTuple!U))
        {
            static if (isTuple!T)
            {
                static assert(distinctFieldNames!(_Fields, T._Fields),
                    "Cannot concatenate tuples with duplicate fields: " ~ fieldNames.stringof ~
                    " - " ~ T.fieldNames.stringof);
                return Tuple!(_Fields, T._Fields)(expand, t.expand);
            }
            else
            {
                return Tuple!(_Fields, T)(expand, t);
            }
        }

        /// ditto
        auto opBinaryRight(string op, T)(auto ref T t)
        if (op == "~" && !(is(T : U[], U) && isTuple!U))
        {
            static if (isTuple!T)
            {
                static assert(distinctFieldNames!(_Fields, T._Fields),
                    "Cannot concatenate tuples with duplicate fields: " ~ T.stringof ~
                    " - " ~ fieldNames.fieldNames.stringof);
                return Tuple!(T._Fields, _Fields)(t.expand, expand);
            }
            else
            {
                return Tuple!(T, _Fields)(t, expand);
            }
        }

        /**
         * Assignment from another `Tuple`.
         *
         * Params:
         *     rhs = The source `Tuple` to assign from. Each element of the
         *           source `Tuple` must be implicitly assignable to each
         *           respective element of the target `Tuple`.
         */
        ref Tuple opAssign(R)(auto ref R rhs)
        if (areCompatibleTuples!(typeof(this), R, "="))
        {
            import std.algorithm.mutation : swap;

            /*
                This optimization caused compilation failures with no error message available:

                > Error: unknown, please file report on issues.dlang.org
                > std/sumtype.d(1262): Error: template instance `std.sumtype.SumType!(Flag, Tuple!(This*))` error instantiating
            */
            version (none)
            {
                static if (is(R == Tuple!Types) && !__traits(isRef, rhs) && isTuple!R)
                {
                    if (__ctfe)
                    {
                        // Cannot use swap at compile time
                        field[] = rhs.field[];
                    }
                    else
                    {
                        // Use swap-and-destroy to optimize rvalue assignment
                        swap!(Tuple!Types)(this, rhs);
                    }
                }
                else
                {
                    // Do not swap; opAssign should be called on the fields.
                    field[] = rhs.field[];
                }
            }

            field[] = rhs.field[];
            return this;
        }

        /**
         * Renames the elements of a $(LREF Tuple).
         *
         * `rename` uses the passed `names` and returns a new
         * $(LREF Tuple) using these names, with the content
         * unchanged.
         * If fewer names are passed than there are members
         * of the $(LREF Tuple) then those trailing members are unchanged.
         * An empty string will remove the name for that member.
         * It is an compile-time error to pass more names than
         * there are members of the $(LREF Tuple).
         */
        ref rename(names...)() inout return
        if (names.length == 0 || allSatisfy!(isSomeString, typeof(names)))
        {
            import std.algorithm.comparison : equal;
            // to circumvent https://issues.dlang.org/show_bug.cgi?id=16418
            static if (names.length == 0 || equal([names], [fieldNames]))
                return this;
            else
            {
                enum nT = Types.length;
                enum nN = names.length;
                static assert(nN <= nT, "Cannot have more names than tuple members");
                alias allNames = AliasSeq!(names, fieldNames[nN .. $]);

                import std.meta : Alias, aliasSeqOf;

                template GetItem(size_t idx)
                {
                    import std.array : empty;
                    static if (idx < nT)
                        alias GetItem = Alias!(Types[idx]);
                    else static if (allNames[idx - nT].empty)
                        alias GetItem = AliasSeq!();
                    else
                        alias GetItem = Alias!(allNames[idx - nT]);
                }

                import std.range : roundRobin, iota;
                alias NewTupleT = Tuple!(staticMap!(GetItem, aliasSeqOf!(
                        roundRobin(iota(nT), iota(nT, 2*nT)))));
                return *(() @trusted => cast(NewTupleT*)&this)();
            }
        }

        ///
        static if (Specs.length == 0) @safe unittest
        {
            auto t0 = tuple(4, "hello");

            auto t0Named = t0.rename!("val", "tag");
            assert(t0Named.val == 4);
            assert(t0Named.tag == "hello");

            Tuple!(float, "dat", size_t[2], "pos") t1;
            t1.pos = [2, 1];
            auto t1Named = t1.rename!"height";
            t1Named.height = 3.4f;
            assert(t1Named.height == 3.4f);
            assert(t1Named.pos == [2, 1]);
            t1Named.rename!"altitude".altitude = 5;
            assert(t1Named.height == 5);

            Tuple!(int, "a", int, int, "c") t2;
            t2 = tuple(3,4,5);
            auto t2Named = t2.rename!("", "b");
            // "a" no longer has a name
            static assert(!__traits(hasMember, typeof(t2Named), "a"));
            assert(t2Named[0] == 3);
            assert(t2Named.b == 4);
            assert(t2Named.c == 5);

            // not allowed to specify more names than the tuple has members
            static assert(!__traits(compiles, t2.rename!("a","b","c","d")));

            // use it in a range pipeline
            import std.range : iota, zip;
            import std.algorithm.iteration : map, sum;
            auto res = zip(iota(1, 4), iota(10, 13))
                .map!(t => t.rename!("a", "b"))
                .map!(t => t.a * t.b)
                .sum;
            assert(res == 68);

            const tup = Tuple!(int, "a", int, "b")(2, 3);
            const renamed = tup.rename!("c", "d");
            assert(renamed.c + renamed.d == 5);
        }

        /**
         * Overload of $(LREF _rename) that takes an associative array
         * `translate` as a template parameter, where the keys are
         * either the names or indices of the members to be changed
         * and the new names are the corresponding values.
         * Every key in `translate` must be the name of a member of the
         * $(LREF tuple).
         * The same rules for empty strings apply as for the variadic
         * template overload of $(LREF _rename).
        */
        ref rename(alias translate)() inout
        if (is(typeof(translate) : V[K], V, K) && isSomeString!V &&
                (isSomeString!K || is(K : size_t)))
        {
            import std.meta : aliasSeqOf;
            import std.range : ElementType;
            static if (isSomeString!(ElementType!(typeof(translate.keys))))
            {
                {
                    import std.conv : to;
                    import std.algorithm.iteration : filter;
                    import std.algorithm.searching : canFind;
                    enum notFound = translate.keys
                        .filter!(k => fieldNames.canFind(k) == -1);
                    static assert(notFound.empty, "Cannot find members "
                        ~ notFound.to!string ~ " in type "
                        ~ typeof(this).stringof);
                }
                return this.rename!(aliasSeqOf!(
                    {
                        import std.array : empty;
                        auto names = [fieldNames];
                        foreach (ref n; names)
                            if (!n.empty)
                                if (auto p = n in translate)
                                    n = *p;
                        return names;
                    }()));
            }
            else
            {
                {
                    import std.algorithm.iteration : filter;
                    import std.conv : to;
                    enum invalid = translate.keys.
                        filter!(k => k < 0 || k >= this.length);
                    static assert(invalid.empty, "Indices " ~ invalid.to!string
                        ~ " are out of bounds for tuple with length "
                        ~ this.length.to!string);
                }
                return this.rename!(aliasSeqOf!(
                    {
                        auto names = [fieldNames];
                        foreach (k, v; translate)
                            names[k] = v;
                        return names;
                    }()));
            }
        }

        ///
        static if (Specs.length == 0) @safe unittest
        {
            //replacing names by their current name

            Tuple!(float, "dat", size_t[2], "pos") t1;
            t1.pos = [2, 1];
            auto t1Named = t1.rename!(["dat": "height"]);
            t1Named.height = 3.4;
            assert(t1Named.pos == [2, 1]);
            t1Named.rename!(["height": "altitude"]).altitude = 5;
            assert(t1Named.height == 5);

            Tuple!(int, "a", int, "b") t2;
            t2 = tuple(3, 4);
            auto t2Named = t2.rename!(["a": "b", "b": "c"]);
            assert(t2Named.b == 3);
            assert(t2Named.c == 4);

            const t3 = Tuple!(int, "a", int, "b")(3, 4);
            const t3Named = t3.rename!(["a": "b", "b": "c"]);
            assert(t3Named.b == 3);
            assert(t3Named.c == 4);
        }

        ///
        static if (Specs.length == 0) @system unittest
        {
            //replace names by their position

            Tuple!(float, "dat", size_t[2], "pos") t1;
            t1.pos = [2, 1];
            auto t1Named = t1.rename!([0: "height"]);
            t1Named.height = 3.4;
            assert(t1Named.pos == [2, 1]);
            t1Named.rename!([0: "altitude"]).altitude = 5;
            assert(t1Named.height == 5);

            Tuple!(int, "a", int, "b", int, "c") t2;
            t2 = tuple(3, 4, 5);
            auto t2Named = t2.rename!([0: "c", 2: "a"]);
            assert(t2Named.a == 5);
            assert(t2Named.b == 4);
            assert(t2Named.c == 3);
        }

        static if (Specs.length == 0) @system unittest
        {
            //check that empty translations work fine
            enum string[string] a0 = null;
            enum string[int] a1 = null;
            Tuple!(float, "a", float, "b") t0;

            auto t1 = t0.rename!a0;

            t1.a = 3;
            t1.b = 4;
            auto t2 = t0.rename!a1;
            t2.a = 3;
            t2.b = 4;
            auto t3 = t0.rename;
            t3.a = 3;
            t3.b = 4;
        }

        /**
         * Takes a slice by-reference of this `Tuple`.
         *
         * Params:
         *     from = A `size_t` designating the starting position of the slice.
         *     to = A `size_t` designating the ending position (exclusive) of the slice.
         *
         * Returns:
         *     A new `Tuple` that is a slice from `[from, to$(RPAREN)` of the original.
         *     It has the same types and values as the range `[from, to$(RPAREN)` in
         *     the original.
         */
        @property
        ref inout(Tuple!(sliceSpecs!(from, to))) slice(size_t from, size_t to)() inout @trusted
        if (from <= to && to <= Types.length)
        {
            static assert(
                (typeof(this).alignof % typeof(return).alignof == 0) &&
                (expand[from].offsetof % typeof(return).alignof == 0),
                "Slicing by reference is impossible because of an alignment mistmatch" ~
                " (See https://issues.dlang.org/show_bug.cgi?id=15645).");

            return *cast(typeof(return)*) &(field[from]);
        }

        ///
        static if (Specs.length == 0) @safe unittest
        {
            Tuple!(int, string, float, double) a;
            a[1] = "abc";
            a[2] = 4.5;
            auto s = a.slice!(1, 3);
            static assert(is(typeof(s) == Tuple!(string, float)));
            assert(s[0] == "abc" && s[1] == 4.5);

            // https://issues.dlang.org/show_bug.cgi?id=15645
            Tuple!(int, short, bool, double) b;
            static assert(!__traits(compiles, b.slice!(2, 4)));
        }

        /**
            Creates a hash of this `Tuple`.

            Returns:
                A `size_t` representing the hash of this `Tuple`.
         */
        size_t toHash() const nothrow @safe
        {
            size_t h = 0;
            static foreach (i, T; Types)
            {{
                static if (__traits(compiles, h = .hashOf(field[i])))
                    const k = .hashOf(field[i]);
                else
                {
                    // Workaround for when .hashOf is not both @safe and nothrow.
                    static if (is(T : shared U, U) && __traits(compiles, (U* a) nothrow @safe => .hashOf(*a))
                            && !__traits(hasMember, T, "toHash"))
                        // BUG: Improperly casts away `shared`!
                        const k = .hashOf(*(() @trusted => cast(U*) &field[i])());
                    else
                        // BUG: Improperly casts away `shared`!
                        const k = typeid(T).getHash((() @trusted => cast(const void*) &field[i])());
                }
                static if (i == 0)
                    h = k;
                else
                    // As in boost::hash_combine
                    // https://www.boost.org/doc/libs/1_55_0/doc/html/hash/reference.html#boost.hash_combine
                    h ^= k + 0x9e3779b9 + (h << 6) + (h >>> 2);
            }}
            return h;
        }

        /**
         * Converts to string.
         *
         * Returns:
         *     The string representation of this `Tuple`.
         */
        string toString()() const
        {
            import std.array : appender;
            auto app = appender!string();
            this.toString((const(char)[] chunk) => app ~= chunk);
            return app.data;
        }

        import std.format.spec : FormatSpec;

        /**
         * Formats `Tuple` with either `%s`, `%(inner%)` or `%(inner%|sep%)`.
         *
         * $(TABLE2 Formats supported by Tuple,
         * $(THEAD Format, Description)
         * $(TROW $(P `%s`), $(P Format like `Tuple!(types)(elements formatted with %s each)`.))
         * $(TROW $(P `%(inner%)`), $(P The format `inner` is applied the expanded `Tuple`$(COMMA) so
         *      it may contain as many formats as the `Tuple` has fields.))
         * $(TROW $(P `%(inner%|sep%)`), $(P The format `inner` is one format$(COMMA) that is applied
         *      on all fields of the `Tuple`. The inner format must be compatible to all
         *      of them.)))
         *
         * Params:
         *     sink = A `char` accepting delegate
         *     fmt = A $(REF FormatSpec, std,format)
         */
        void toString(DG)(scope DG sink) const
        {
            auto f = FormatSpec!char();
            toString(sink, f);
        }

        /// ditto
        void toString(DG, Char)(scope DG sink, scope const ref FormatSpec!Char fmt) const
        {
            import std.format : format, FormatException;
            import std.format.write : formattedWrite;
            import std.range : only;
            if (fmt.nested)
            {
                if (fmt.sep)
                {
                    foreach (i, Type; Types)
                    {
                        static if (i > 0)
                        {
                            sink(fmt.sep);
                        }
                        // TODO: Change this once formattedWrite() works for shared objects.
                        static if (is(Type == class) && is(Type == shared))
                        {
                            sink(Type.stringof);
                        }
                        else
                        {
                            formattedWrite(sink, fmt.nested, this.field[i]);
                        }
                    }
                }
                else
                {
                    formattedWrite(sink, fmt.nested, staticMap!(sharedToString, this.expand));
                }
            }
            else if (fmt.spec == 's')
            {
                enum header = Unqual!(typeof(this)).stringof ~ "(",
                     footer = ")",
                     separator = ", ";
                sink(header);
                foreach (i, Type; Types)
                {
                    static if (i > 0)
                    {
                        sink(separator);
                    }
                    // TODO: Change this once format() works for shared objects.
                    static if (is(Type == class) && is(Type == shared))
                    {
                        sink(Type.stringof);
                    }
                    else
                    {
                        sink(format!("%(%s%)")(only(field[i])));
                    }
                }
                sink(footer);
            }
            else
            {
                const spec = fmt.spec;
                throw new FormatException(
                    "Expected '%s' or '%(...%)' or '%(...%|...%)' format specifier for type '" ~
                        Unqual!(typeof(this)).stringof ~ "', not '%" ~ spec ~ "'.");
            }
        }

        ///
        static if (Specs.length == 0) @safe unittest
        {
            import std.format : format;

            Tuple!(int, double)[3] tupList = [ tuple(1, 1.0), tuple(2, 4.0), tuple(3, 9.0) ];

            // Default format
            assert(format("%s", tuple("a", 1)) == `Tuple!(string, int)("a", 1)`);

            // One Format for each individual component
            assert(format("%(%#x v %.4f w %#x%)", tuple(1, 1.0, 10))         == `0x1 v 1.0000 w 0xa`);
            assert(format(  "%#x v %.4f w %#x"  , tuple(1, 1.0, 10).expand)  == `0x1 v 1.0000 w 0xa`);

            // One Format for all components
            assert(format("%(>%s<%| & %)", tuple("abc", 1, 2.3, [4, 5])) == `>abc< & >1< & >2.3< & >[4, 5]<`);

            // Array of Tuples
            assert(format("%(%(f(%d) = %.1f%);  %)", tupList) == `f(1) = 1.0;  f(2) = 4.0;  f(3) = 9.0`);
        }

        ///
        static if (Specs.length == 0) @safe unittest
        {
            import std.exception : assertThrown;
            import std.format : format, FormatException;

            // Error: %( %) missing.
            assertThrown!FormatException(
                format("%d, %f", tuple(1, 2.0)) == `1, 2.0`
            );

            // Error: %( %| %) missing.
            assertThrown!FormatException(
                format("%d", tuple(1, 2)) == `1, 2`
            );

            // Error: %d inadequate for double
            assertThrown!FormatException(
                format("%(%d%|, %)", tuple(1, 2.0)) == `1, 2.0`
            );
        }
    }
}

///
@safe unittest
{
    Tuple!(int, int) point;
    // assign coordinates
    point[0] = 5;
    point[1] = 6;
    // read coordinates
    auto x = point[0];
    auto y = point[1];
}

/**
    `Tuple` members can be named. It is legal to mix named and unnamed
    members. The method above is still applicable to all fields.
 */
@safe unittest
{
    alias Entry = Tuple!(int, "index", string, "value");
    Entry e;
    e.index = 4;
    e.value = "Hello";
    assert(e[1] == "Hello");
    assert(e[0] == 4);
}

/**
    A `Tuple` with named fields is a distinct type from a `Tuple` with unnamed
    fields, i.e. each naming imparts a separate type for the `Tuple`. Two
    `Tuple`s differing in naming only are still distinct, even though they
    might have the same structure.
 */
@safe unittest
{
    Tuple!(int, "x", int, "y") point1;
    Tuple!(int, int) point2;
    assert(!is(typeof(point1) == typeof(point2)));
}

/// Use tuples as ranges
@safe unittest
{
    import std.algorithm.iteration : sum;
    import std.range : only;
    auto t = tuple(1, 2);
    assert(t.expand.only.sum == 3);
}

// https://issues.dlang.org/show_bug.cgi?id=4582
@safe unittest
{
    static assert(!__traits(compiles, Tuple!(string, "id", int, "id")));
    static assert(!__traits(compiles, Tuple!(string, "str", int, "i", string, "str", float)));
}

/// Concatenate tuples
@safe unittest
{
    import std.meta : AliasSeq;
    auto t = tuple(1, "2") ~ tuple(ushort(42), true);
    static assert(is(t.Types == AliasSeq!(int, string, ushort, bool)));
    assert(t[1] == "2");
    assert(t[2] == 42);
    assert(t[3] == true);
}

// https://issues.dlang.org/show_bug.cgi?id=14637
// tuple concat
@safe unittest
{
    auto t = tuple!"foo"(1.0) ~ tuple!"bar"("3");
    static assert(is(t.Types == AliasSeq!(double, string)));
    static assert(t.fieldNames == tuple("foo", "bar"));
    assert(t.foo == 1.0);
    assert(t.bar == "3");
}

// https://issues.dlang.org/show_bug.cgi?id=18824
// tuple concat
@safe unittest
{
    alias Type = Tuple!(int, string);
    Type[] arr;
    auto t = tuple(2, "s");
    // Test opBinaryRight
    arr = arr ~ t;
    // Test opBinary
    arr = t ~ arr;
    static assert(is(typeof(arr) == Type[]));
    immutable Type[] b;
    auto c = b ~ t;
    static assert(is(typeof(c) == immutable(Type)[]));
}

// tuple concat
@safe unittest
{
    auto t = tuple!"foo"(1.0) ~ "3";
    static assert(is(t.Types == AliasSeq!(double, string)));
    assert(t.foo == 1.0);
    assert(t[1]== "3");
}

// tuple concat
@safe unittest
{
    auto t = "2" ~ tuple!"foo"(1.0);
    static assert(is(t.Types == AliasSeq!(string, double)));
    assert(t.foo == 1.0);
    assert(t[0]== "2");
}

// tuple concat
@safe unittest
{
    auto t = "2" ~ tuple!"foo"(1.0) ~ tuple(42, 3.0f) ~ real(1) ~ "a";
    static assert(is(t.Types == AliasSeq!(string, double, int, float, real, string)));
    assert(t.foo == 1.0);
    assert(t[0] == "2");
    assert(t[1] == 1.0);
    assert(t[2] == 42);
    assert(t[3] == 3.0f);
    assert(t[4] == 1.0);
    assert(t[5] == "a");
}

// ensure that concatenation of tuples with non-distinct fields is forbidden
@safe unittest
{
    static assert(!__traits(compiles,
        tuple!("a")(0) ~ tuple!("a")("1")));
    static assert(!__traits(compiles,
        tuple!("a", "b")(0, 1) ~ tuple!("b", "a")("3", 1)));
    static assert(!__traits(compiles,
        tuple!("a")(0) ~ tuple!("b", "a")("3", 1)));
    static assert(!__traits(compiles,
        tuple!("a1", "a")(1.0, 0) ~ tuple!("a2", "a")("3", 0)));
}

// Ensure that Tuple comparison with non-const opEquals works
@safe unittest
{
    static struct Bad
    {
        int a;

        bool opEquals(Bad b)
        {
            return a == b.a;
        }
    }

    auto t = Tuple!(int, Bad, string)(1, Bad(1), "asdf");

    //Error: mutable method Bad.opEquals is not callable using a const object
    assert(t == AliasSeq!(1, Bad(1), "asdf"));
}

// Ensure Tuple.toHash works
@safe unittest
{
    Tuple!(int, int) point;
    assert(point.toHash == typeof(point).init.toHash);
    assert(tuple(1, 2) != point);
    assert(tuple(1, 2) == tuple(1, 2));
    point[0] = 1;
    assert(tuple(1, 2) != point);
    point[1] = 2;
    assert(tuple(1, 2) == point);
}

@safe @betterC unittest
{
    auto t = tuple(1, 2);
    assert(t == tuple(1, 2));
    auto t3 = tuple(1, 'd');
}

// https://issues.dlang.org/show_bug.cgi?id=20850
// Assignment to enum tuple
@safe unittest
{
    enum T : Tuple!(int*) { a = T(null) }
    T t;
    t = T.a;
}

// https://issues.dlang.org/show_bug.cgi?id=13663
@safe unittest
{
    auto t = tuple(real.nan);
    assert(!(t > t));
    assert(!(t < t));
    assert(!(t == t));
}

@safe unittest
{
    struct S
    {
        float opCmp(S s) { return float.nan; }
        bool opEquals(S s) { return false; }
    }

    auto t = tuple(S());
    assert(!(t > t));
    assert(!(t < t));
    assert(!(t == t));
}

// https://issues.dlang.org/show_bug.cgi?id=8015
@safe unittest
{
    struct MyStruct
    {
        string str;
        @property string toStr()
        {
            return str;
        }
        alias toStr this;
    }

    Tuple!(MyStruct) t;
}

// https://issues.dlang.org/show_bug.cgi?id=24465
@safe unittest
{
    {
        static struct S
        {
            this(ref return scope inout(S) rhs) scope @trusted inout pure nothrow {}
        }

        static void foo(Tuple!S)
        {
        }

        Tuple!S t;
        foo(t);

        auto t2 = Tuple!S(t);
    }

    {
        static struct S {}
        Tuple!S t;
        auto t2 = Tuple!S(t);

        // This can't be done if Tuple has a copy constructor, because it's not
        // allowed to have an rvalue constructor at that point, and the
        // compiler doesn't to something intelligent like transform it into a
        // move instead. However, it has been legal with Tuple for a while
        // (maybe even since it was first added) when the type doesn't have a
        // copy constructor, so this is testing to make sure that the fix to
        // make copy constructors work doesn't mess up the rvalue constructor
        // when none of the Tuple's members have copy constructors.
        auto t3 = Tuple!S(Tuple!S.init);
    }
}

/**
    Creates a copy of a $(LREF Tuple) with its fields in _reverse order.

    Params:
        t = The `Tuple` to copy.

    Returns:
        A new `Tuple`.
 */
auto reverse(T)(T t)
if (isTuple!T)
{
    import std.meta : Reverse;
    // @@@BUG@@@ Cannot be an internal function due to forward reference issues.

    // @@@BUG@@@ 9929 Need 'this' when calling template with expanded tuple
    // return tuple(Reverse!(t.expand));

    ReverseTupleType!T result;
    auto tup = t.expand;
    result.expand = Reverse!tup;
    return result;
}

///
@safe unittest
{
    auto tup = tuple(1, "2");
    assert(tup.reverse == tuple("2", 1));
}

/* Get a Tuple type with the reverse specification of Tuple T. */
private template ReverseTupleType(T)
if (isTuple!T)
{
    static if (is(T : Tuple!A, A...))
        alias ReverseTupleType = Tuple!(ReverseTupleSpecs!A);
}

/* Reverse the Specs of a Tuple. */
private template ReverseTupleSpecs(T...)
{
    static if (T.length > 1)
    {
        static if (is(typeof(T[$-1]) : string))
        {
            alias ReverseTupleSpecs = AliasSeq!(T[$-2], T[$-1], ReverseTupleSpecs!(T[0 .. $-2]));
        }
        else
        {
            alias ReverseTupleSpecs = AliasSeq!(T[$-1], ReverseTupleSpecs!(T[0 .. $-1]));
        }
    }
    else
    {
        alias ReverseTupleSpecs = T;
    }
}

// ensure that internal Tuple unittests are compiled
@safe unittest
{
    Tuple!() t;
}

@safe unittest
{
    import std.conv;
    {
        Tuple!(int, "a", int, "b") nosh;
        static assert(nosh.length == 2);
        nosh.a = 5;
        nosh.b = 6;
        assert(nosh.a == 5);
        assert(nosh.b == 6);
    }
    {
        Tuple!(short, double) b;
        static assert(b.length == 2);
        b[1] = 5;
        auto a = Tuple!(int, real)(b);
        assert(a[0] == 0 && a[1] == 5);
        a = Tuple!(int, real)(1, 2);
        assert(a[0] == 1 && a[1] == 2);
        auto c = Tuple!(int, "a", double, "b")(a);
        assert(c[0] == 1 && c[1] == 2);
    }
    {
        Tuple!(int, real) nosh;
        nosh[0] = 5;
        nosh[1] = 0;
        assert(nosh[0] == 5 && nosh[1] == 0);
        assert(nosh.to!string == "Tuple!(int, real)(5, 0)", nosh.to!string);
        Tuple!(int, int) yessh;
        nosh = yessh;
    }
    {
        class A {}
        Tuple!(int, shared A) nosh;
        nosh[0] = 5;
        assert(nosh[0] == 5 && nosh[1] is null);
        assert(nosh.to!string == "Tuple!(int, shared(A))(5, shared(A))");
    }
    {
        Tuple!(int, string) t;
        t[0] = 10;
        t[1] = "str";
        assert(t[0] == 10 && t[1] == "str");
        assert(t.to!string == `Tuple!(int, string)(10, "str")`, t.to!string);
    }
    {
        Tuple!(int, "a", double, "b") x;
        static assert(x.a.offsetof == x[0].offsetof);
        static assert(x.b.offsetof == x[1].offsetof);
        x.b = 4.5;
        x.a = 5;
        assert(x[0] == 5 && x[1] == 4.5);
        assert(x.a == 5 && x.b == 4.5);
    }
    // indexing
    {
        Tuple!(int, real) t;
        static assert(is(typeof(t[0]) == int));
        static assert(is(typeof(t[1]) == real));
        int* p0 = &t[0];
        real* p1 = &t[1];
        t[0] = 10;
        t[1] = -200.0L;
        assert(*p0 == t[0]);
        assert(*p1 == t[1]);
    }
    // slicing
    {
        Tuple!(int, "x", real, "y", double, "z", string) t;
        t[0] = 10;
        t[1] = 11;
        t[2] = 12;
        t[3] = "abc";
        auto a = t.slice!(0, 3);
        assert(a.length == 3);
        assert(a.x == t.x);
        assert(a.y == t.y);
        assert(a.z == t.z);
        auto b = t.slice!(2, 4);
        assert(b.length == 2);
        assert(b.z == t.z);
        assert(b[1] == t[3]);
    }
    // nesting
    {
        Tuple!(Tuple!(int, real), Tuple!(string, "s")) t;
        static assert(is(typeof(t[0]) == Tuple!(int, real)));
        static assert(is(typeof(t[1]) == Tuple!(string, "s")));
        static assert(is(typeof(t[0][0]) == int));
        static assert(is(typeof(t[0][1]) == real));
        static assert(is(typeof(t[1].s) == string));
        t[0] = tuple(10, 20.0L);
        t[1].s = "abc";
        assert(t[0][0] == 10);
        assert(t[0][1] == 20.0L);
        assert(t[1].s == "abc");
    }
    // non-POD
    {
        static struct S
        {
            int count;
            this(this) { ++count; }
            ~this() { --count; }
            void opAssign(S rhs) { count = rhs.count; }
        }
        Tuple!(S, S) ss;
        Tuple!(S, S) ssCopy = ss;
        assert(ssCopy[0].count == 1);
        assert(ssCopy[1].count == 1);
        ssCopy[1] = ssCopy[0];
        assert(ssCopy[1].count == 2);
    }
    // https://issues.dlang.org/show_bug.cgi?id=2800
    {
        static struct R
        {
            Tuple!(int, int) _front;
            @property ref Tuple!(int, int) front() return { return _front;  }
            @property bool empty() { return _front[0] >= 10; }
            void popFront() { ++_front[0]; }
        }
        foreach (a; R())
        {
            static assert(is(typeof(a) == Tuple!(int, int)));
            assert(0 <= a[0] && a[0] < 10);
            assert(a[1] == 0);
        }
    }
    // Construction with compatible elements
    {
        auto t1 = Tuple!(int, double)(1, 1);

        // https://issues.dlang.org/show_bug.cgi?id=8702
        auto t8702a = tuple(tuple(1));
        auto t8702b = Tuple!(Tuple!(int))(Tuple!(int)(1));
    }
    // Construction with compatible tuple
    {
        Tuple!(int, int) x;
        x[0] = 10;
        x[1] = 20;
        Tuple!(int, "a", double, "b") y = x;
        assert(y.a == 10);
        assert(y.b == 20);
        // incompatible
        static assert(!__traits(compiles, Tuple!(int, int)(y)));
    }
    // https://issues.dlang.org/show_bug.cgi?id=6275
    {
        const int x = 1;
        auto t1 = tuple(x);
        alias T = Tuple!(const(int));
        auto t2 = T(1);
    }
    // https://issues.dlang.org/show_bug.cgi?id=9431
    {
        alias T = Tuple!(int[1][]);
        auto t = T([[10]]);
    }
    // https://issues.dlang.org/show_bug.cgi?id=7666
    {
        auto tup = tuple(1, "2");
        assert(tup.reverse == tuple("2", 1));
    }
    {
        Tuple!(int, "x", string, "y") tup = tuple(1, "2");
        auto rev = tup.reverse;
        assert(rev == tuple("2", 1));
        assert(rev.x == 1 && rev.y == "2");
    }
    {
        Tuple!(wchar, dchar, int, "x", string, "y", char, byte, float) tup;
        tup = tuple('a', 'b', 3, "4", 'c', cast(byte) 0x0D, 0.00);
        auto rev = tup.reverse;
        assert(rev == tuple(0.00, cast(byte) 0x0D, 'c', "4", 3, 'b', 'a'));
        assert(rev.x == 3 && rev.y == "4");
    }
}
@safe unittest
{
    // opEquals
    {
        struct Equ1 { bool opEquals(Equ1) { return true; } }
        auto  tm1 = tuple(Equ1.init);
        const tc1 = tuple(Equ1.init);
        static assert( is(typeof(tm1 == tm1)));
        static assert(!is(typeof(tm1 == tc1)));
        static assert(!is(typeof(tc1 == tm1)));
        static assert(!is(typeof(tc1 == tc1)));

        struct Equ2 { bool opEquals(const Equ2) const { return true; } }
        auto  tm2 = tuple(Equ2.init);
        const tc2 = tuple(Equ2.init);
        static assert( is(typeof(tm2 == tm2)));
        static assert( is(typeof(tm2 == tc2)));
        static assert( is(typeof(tc2 == tm2)));
        static assert( is(typeof(tc2 == tc2)));

        // https://issues.dlang.org/show_bug.cgi?id=8686
        struct Equ3 { bool opEquals(T)(T) { return true; } }
        auto  tm3 = tuple(Equ3.init);
        const tc3 = tuple(Equ3.init);
        static assert( is(typeof(tm3 == tm3)));
        static assert( is(typeof(tm3 == tc3)));
        static assert(!is(typeof(tc3 == tm3)));
        static assert(!is(typeof(tc3 == tc3)));

        struct Equ4 { bool opEquals(T)(T) const { return true; } }
        auto  tm4 = tuple(Equ4.init);
        const tc4 = tuple(Equ4.init);
        static assert( is(typeof(tm4 == tm4)));
        static assert( is(typeof(tm4 == tc4)));
        static assert( is(typeof(tc4 == tm4)));
        static assert( is(typeof(tc4 == tc4)));
    }
    // opCmp
    {
        struct Cmp1 { int opCmp(Cmp1) { return 0; } }
        auto  tm1 = tuple(Cmp1.init);
        const tc1 = tuple(Cmp1.init);
        static assert( is(typeof(tm1 < tm1)));
        static assert(!is(typeof(tm1 < tc1)));
        static assert(!is(typeof(tc1 < tm1)));
        static assert(!is(typeof(tc1 < tc1)));

        struct Cmp2 { int opCmp(const Cmp2) const { return 0; } }
        auto  tm2 = tuple(Cmp2.init);
        const tc2 = tuple(Cmp2.init);
        static assert( is(typeof(tm2 < tm2)));
        static assert( is(typeof(tm2 < tc2)));
        static assert( is(typeof(tc2 < tm2)));
        static assert( is(typeof(tc2 < tc2)));

        struct Cmp3 { int opCmp(T)(T) { return 0; } }
        auto  tm3 = tuple(Cmp3.init);
        const tc3 = tuple(Cmp3.init);
        static assert( is(typeof(tm3 < tm3)));
        static assert( is(typeof(tm3 < tc3)));
        static assert(!is(typeof(tc3 < tm3)));
        static assert(!is(typeof(tc3 < tc3)));

        struct Cmp4 { int opCmp(T)(T) const { return 0; } }
        auto  tm4 = tuple(Cmp4.init);
        const tc4 = tuple(Cmp4.init);
        static assert( is(typeof(tm4 < tm4)));
        static assert( is(typeof(tm4 < tc4)));
        static assert( is(typeof(tc4 < tm4)));
        static assert( is(typeof(tc4 < tc4)));
    }
    // https://issues.dlang.org/show_bug.cgi?id=14890
    static void test14890(inout int[] dummy)
    {
        alias V = Tuple!(int, int);

                    V mv;
              const V cv;
          immutable V iv;
              inout V wv;   // OK <- NG
        inout const V wcv;  // OK <- NG

        static foreach (v1; AliasSeq!(mv, cv, iv, wv, wcv))
        static foreach (v2; AliasSeq!(mv, cv, iv, wv, wcv))
        {
            assert(!(v1 < v2));
        }
    }
    {
        int[2] ints = [ 1, 2 ];
        Tuple!(int, int) t = ints;
        assert(t[0] == 1 && t[1] == 2);
        Tuple!(long, uint) t2 = ints;
        assert(t2[0] == 1 && t2[1] == 2);
    }
}
@safe unittest
{
    auto t1 = Tuple!(int, "x", string, "y")(1, "a");
    assert(t1.x == 1);
    assert(t1.y == "a");
    void foo(Tuple!(int, string) t2) {}
    foo(t1);

    Tuple!(int, int)[] arr;
    arr ~= tuple(10, 20); // OK
    arr ~= Tuple!(int, "x", int, "y")(10, 20); // NG -> OK

    static assert(is(typeof(Tuple!(int, "x", string, "y").tupleof) ==
                     typeof(Tuple!(int,      string     ).tupleof)));
}
@safe unittest
{
    // https://issues.dlang.org/show_bug.cgi?id=10686
    immutable Tuple!(int) t1;
    auto r1 = t1[0]; // OK
    immutable Tuple!(int, "x") t2;
    auto r2 = t2[0]; // error
}
@safe unittest
{
    import std.exception : assertCTFEable;

    // https://issues.dlang.org/show_bug.cgi?id=10218
    assertCTFEable!(
    {
        auto t = tuple(1);
        t = tuple(2);   // assignment
    });
}
@safe unittest
{
    class Foo{}
    Tuple!(immutable(Foo)[]) a;
}

@safe unittest
{
    //Test non-assignable
    static struct S
    {
        int* p;
    }
    alias IS = immutable S;
    static assert(!isAssignable!IS);

    auto s = IS.init;

    alias TIS = Tuple!IS;
    TIS a = tuple(s);
    TIS b = a;

    alias TISIS = Tuple!(IS, IS);
    TISIS d = tuple(s, s);
    IS[2] ss;
    TISIS e = TISIS(ss);
}

// https://issues.dlang.org/show_bug.cgi?id=9819
@safe unittest
{
    alias T = Tuple!(int, "x", double, "foo");
    static assert(T.fieldNames[0] == "x");
    static assert(T.fieldNames[1] == "foo");

    alias Fields = Tuple!(int, "id", string, float);
    static assert(Fields.fieldNames == AliasSeq!("id", "", ""));
}

// https://issues.dlang.org/show_bug.cgi?id=13837
@safe unittest
{
    // New behaviour, named arguments.
    static assert(is(
        typeof(tuple!("x")(1)) == Tuple!(int, "x")));
    static assert(is(
        typeof(tuple!("x")(1.0)) == Tuple!(double, "x")));
    static assert(is(
        typeof(tuple!("x")("foo")) == Tuple!(string, "x")));
    static assert(is(
        typeof(tuple!("x", "y")(1, 2.0)) == Tuple!(int, "x", double, "y")));

    auto a = tuple!("a", "b", "c")("1", 2, 3.0f);
    static assert(is(typeof(a.a) == string));
    static assert(is(typeof(a.b) == int));
    static assert(is(typeof(a.c) == float));

    // Old behaviour, but with explicit type parameters.
    static assert(is(
        typeof(tuple!(int, double)(1, 2.0)) == Tuple!(int, double)));
    static assert(is(
        typeof(tuple!(const int)(1)) == Tuple!(const int)));
    static assert(is(
        typeof(tuple()) == Tuple!()));

    // Nonsensical behaviour
    static assert(!__traits(compiles, tuple!(1)(2)));
    static assert(!__traits(compiles, tuple!("x")(1, 2)));
    static assert(!__traits(compiles, tuple!("x", "y")(1)));
    static assert(!__traits(compiles, tuple!("x")()));
    static assert(!__traits(compiles, tuple!("x", int)(2)));
}

@safe unittest
{
    class C { override size_t toHash() const nothrow @safe { return 0; } }
    Tuple!(Rebindable!(const C)) a;
    Tuple!(const C) b;
    a = b;
}

@nogc @safe unittest
{
    alias T = Tuple!(string, "s");
    T x;
    x = T.init;
}

@safe unittest
{
    import std.format : format, FormatException;
    import std.exception : assertThrown;

    //enum tupStr = tuple(1, 1.0).toString; // toString is *impure*.
    //static assert(tupStr == `Tuple!(int, double)(1, 1)`);
}

// https://issues.dlang.org/show_bug.cgi?id=17803, parte uno
@safe unittest
{
    auto a = tuple(3, "foo");
    assert(__traits(compiles, { a = (a = a); }));
}
// Ditto
@safe unittest
{
    Tuple!(int[]) a, b, c;
    a = tuple([0, 1, 2]);
    c = b = a;
    assert(a[0].length == b[0].length && b[0].length == c[0].length);
    assert(a[0].ptr == b[0].ptr && b[0].ptr == c[0].ptr);
}

/**
    Constructs a $(LREF Tuple) object instantiated and initialized according to
    the given arguments.

    Params:
        Names = An optional list of strings naming each successive field of the `Tuple`
                or a list of types that the elements are being casted to.
                For a list of names,
                each name matches up with the corresponding field given by `Args`.
                A name does not have to be provided for every field, but as
                the names must proceed in order, it is not possible to skip
                one field and name the next after it.
                For a list of types,
                there must be exactly as many types as parameters.
*/
template tuple(Names...)
{
    /**
    Params:
        args = Values to initialize the `Tuple` with. The `Tuple`'s type will
               be inferred from the types of the values given.

    Returns:
        A new `Tuple` with its type inferred from the arguments given.
     */
    auto tuple(Args...)(Args args)
    {
        static if (Names.length == 0)
        {
            // No specified names, just infer types from Args...
            return Tuple!Args(args);
        }
        else static if (!is(typeof(Names[0]) : string))
        {
            // Names[0] isn't a string, must be explicit types.
            return Tuple!Names(args);
        }
        else
        {
            // Names[0] is a string, so must be specifying names.
            static assert(Names.length == Args.length,
                "Insufficient number of names given.");

            // Interleave(a, b).and(c, d) == (a, c, b, d)
            // This is to get the interleaving of types and names for Tuple
            // e.g. Tuple!(int, "x", string, "y")
            template Interleave(A...)
            {
                template and(B...)
                if (B.length == 1)
                {
                    alias and = AliasSeq!(A[0], B[0]);
                }

                template and(B...)
                if (B.length != 1)
                {
                    alias and = AliasSeq!(A[0], B[0],
                        Interleave!(A[1..$]).and!(B[1..$]));
                }
            }
            return Tuple!(Interleave!(Args).and!(Names))(args);
        }
    }
}

///
@safe unittest
{
    auto value = tuple(5, 6.7, "hello");
    assert(value[0] == 5);
    assert(value[1] == 6.7);
    assert(value[2] == "hello");

    // Field names can be provided.
    auto entry = tuple!("index", "value")(4, "Hello");
    assert(entry.index == 4);
    assert(entry.value == "Hello");
}

/**
    Returns `true` if and only if `T` is an instance of `std.typecons.Tuple`.

    Params:
        T = The type to check.

    Returns:
        true if `T` is a `Tuple` type, false otherwise.
 */
enum isTuple(T) = __traits(compiles,
                           {
                               void f(Specs...)(Tuple!Specs tup) {}
                               f(T.init);
                           } );

///
@safe unittest
{
    static assert(isTuple!(Tuple!()));
    static assert(isTuple!(Tuple!(int)));
    static assert(isTuple!(Tuple!(int, real, string)));
    static assert(isTuple!(Tuple!(int, "x", real, "y")));
    static assert(isTuple!(Tuple!(int, Tuple!(real), string)));
}

@safe unittest
{
    static assert(isTuple!(const Tuple!(int)));
    static assert(isTuple!(immutable Tuple!(int)));

    static assert(!isTuple!(int));
    static assert(!isTuple!(const int));

    struct S {}
    static assert(!isTuple!(S));
}

// used by both Rebindable and UnqualRef
private mixin template RebindableCommon(T, U, alias This)
if (is(T == class) || is(T == interface) || isAssociativeArray!T)
{
    private union
    {
        T original;
        U stripped;
    }

    void opAssign(return scope T another) pure nothrow @nogc
    {
        // If `T` defines `opCast` we must infer the safety
        static if (hasMember!(T, "opCast"))
        {
            // This will allow the compiler to infer the safety of `T.opCast!U`
            // without generating any runtime cost
            if (false) { stripped = cast(U) another; }
        }
        () @trusted { stripped = cast(U) another; }();
    }

    void opAssign(typeof(this) another) @trusted pure nothrow @nogc
    {
        stripped = another.stripped;
    }

    static if (is(T == const U) && is(T == const shared U))
    {
        // safely assign immutable to const / const shared
        void opAssign(This!(immutable U) another) @trusted pure nothrow @nogc
        {
            stripped = another.stripped;
        }
    }

    this(T initializer) pure nothrow @nogc
    {
        // Infer safety from opAssign
        opAssign(initializer);
    }

    @property inout(T) get() @trusted pure nothrow @nogc return scope inout
    {
        return original;
    }

    bool opEquals()(auto ref const(typeof(this)) rhs) const
    {
        // Must forward explicitly because 'stripped' is part of a union.
        // The necessary 'toHash' is forwarded to the class via alias this.
        return stripped == rhs.stripped;
    }

    bool opEquals(const(U) rhs) const
    {
        return stripped == rhs;
    }

    alias get this;
}

/**
`Rebindable!(T)` is a simple, efficient wrapper that behaves just
like an object of type `T`, except that you can reassign it to
refer to another object. For completeness, `Rebindable!(T)` aliases
itself away to `T` if `T` is a non-const object type.

You may want to use `Rebindable` when you want to have mutable
storage referring to `const` objects, for example an array of
references that must be sorted in place. `Rebindable` does not
break the soundness of D's type system and does not incur any of the
risks usually associated with `cast`.

Params:
    T = Any type.
 */
template Rebindable(T)
if (is(T == class) || is(T == interface) || isDynamicArray!T || isAssociativeArray!T)
{
    static if (is(T == const U, U) || is(T == immutable U, U))
    {
        static if (isDynamicArray!T)
        {
            import std.range.primitives : ElementEncodingType;
            alias Rebindable = const(ElementEncodingType!T)[];
        }
        else
        {
            struct Rebindable
            {
                mixin RebindableCommon!(T, U, Rebindable);
            }
        }
    }
    else
    {
        alias Rebindable = T;
    }
}

///Regular `const` object references cannot be reassigned.
@safe unittest
{
    class Widget { int x; int y() @safe const { return x; } }
    const a = new Widget;
    // Fine
    a.y();
    // error! can't modify const a
    // a.x = 5;
    // error! can't modify const a
    // a = new Widget;
}

/**
    However, `Rebindable!(Widget)` does allow reassignment,
    while otherwise behaving exactly like a $(D const Widget).
 */
@safe unittest
{
    class Widget { int x; int y() const @safe { return x; } }
    auto a = Rebindable!(const Widget)(new Widget);
    // Fine
    a.y();
    // error! can't modify const a
    // a.x = 5;
    // Fine
    a = new Widget;
}

// https://issues.dlang.org/show_bug.cgi?id=16054
@safe unittest
{
    Rebindable!(immutable Object) r;
    static assert(__traits(compiles, r.get()));
    static assert(!__traits(compiles, &r.get()));
}

/// ditto
struct Rebindable(T)
if (!is(T == class) && !is(T == interface) && !isDynamicArray!T && !isAssociativeArray!T)
{
private:
    static if (isAssignable!(typeof(cast() T.init)))
    {
        enum useQualifierCast = true;

        typeof(cast() T.init) data;
    }
    else
    {
        enum useQualifierCast = false;

        align(T.alignof)
        static struct Payload
        {
            static if (hasIndirections!T)
            {
                void[T.sizeof] data;
            }
            else
            {
                ubyte[T.sizeof] data;
            }
        }

        Payload data;
    }

public:

    static if (!__traits(compiles, { T value; }))
    {
        @disable this();
    }

    /**
     * Constructs a `Rebindable` from a given value.
     */
    this(T value) @trusted
    {
        static if (useQualifierCast)
        {
            this.data = cast() value;
        }
        else
        {
            set(value);
        }
    }

    /**
     * Overwrites the currently stored value with `value`.
     */
    void opAssign(this This)(T value) @trusted
    {
        clear;
        set(value);
    }

    /**
     * Returns the value currently stored in the `Rebindable`.
     */
    T get(this This)() @property @trusted
    {
        static if (useQualifierCast)
        {
            return cast(T) this.data;
        }
        else
        {
            return *cast(T*) &this.data;
        }
    }

    static if (!useQualifierCast)
    {
        ~this() @trusted
        {
            clear;
        }
    }

    ///
    alias get this;

private:

    void set(this This)(T value)
    {
        static if (useQualifierCast)
        {
            this.data = cast() value;
        }
        else
        {
            // As we're escaping a copy of `value`, deliberately leak a copy:
            static union DontCallDestructor
            {
                T value;
            }
            DontCallDestructor copy = DontCallDestructor(value);
            this.data = *cast(Payload*) &copy;
        }
    }

    void clear(this This)()
    {
        // work around reinterpreting cast being impossible in CTFE
        if (__ctfe)
        {
            return;
        }

        // call possible struct destructors
        .destroy!(No.initialize)(*cast(T*) &this.data);
    }
}

/// Using Rebindable in a generic algorithm:
@safe unittest
{
    import std.range.primitives : front, popFront;

    // simple version of std.algorithm.searching.maxElement
    typeof(R.init.front) maxElement(R)(R r)
    {
        auto max = rebindable(r.front);
        r.popFront;
        foreach (e; r)
            if (e > max)
                max = e; // Rebindable allows const-correct reassignment
        return max;
    }
    struct S
    {
        char[] arr;
        alias arr this; // for comparison
    }
    // can't convert to mutable
    const S cs;
    static assert(!__traits(compiles, { S s = cs; }));

    alias CS = const S;
    CS[] arr = [CS("harp"), CS("apple"), CS("pot")];
    CS ms = maxElement(arr);
    assert(ms.arr == "pot");
}

// https://issues.dlang.org/show_bug.cgi?id=18615
// Rebindable!A should use A.opEqualsa
@system unittest
{
    class CustomOpEq
    {
        int x;
        override bool opEquals(Object rhsObj)
        {
            if (auto rhs = cast(const(CustomOpEq)) rhsObj)
                return this.x == rhs.x;
            else
                return false;
        }
    }
    CustomOpEq a = new CustomOpEq();
    CustomOpEq b = new CustomOpEq();
    assert(a !is b);
    assert(a == b, "a.x == b.x should be true (0 == 0).");

    Rebindable!(const(CustomOpEq)) ra = a;
    Rebindable!(const(CustomOpEq)) rb = b;
    assert(ra !is rb);
    assert(ra == rb, "Rebindable should use CustomOpEq's opEquals, not 'is'.");
    assert(ra == b, "Rebindable!(someQualifier(A)) should be comparable"
        ~ " against const(A) via A.opEquals.");
    assert(a == rb, "Rebindable!(someQualifier(A)) should be comparable"
        ~ " against const(A) via A.opEquals.");

    b.x = 1;
    assert(a != b);
    assert(ra != b, "Rebindable!(someQualifier(A)) should be comparable"
        ~ " against const(A) via A.opEquals.");
    assert(a != rb, "Rebindable!(someQualifier(A)) should be comparable"
        ~ " against const(A) via A.opEquals.");

    Rebindable!(const(Object)) o1 = new Object();
    Rebindable!(const(Object)) o2 = new Object();
    assert(o1 !is o2);
    assert(o1 == o1, "When the class doesn't provide its own opEquals,"
        ~ " Rebindable treats 'a == b' as 'a is b' like Object.opEquals.");
    assert(o1 != o2, "When the class doesn't provide its own opEquals,"
        ~ " Rebindable treats 'a == b' as 'a is b' like Object.opEquals.");
    assert(o1 != new Object(), "Rebindable!(const(Object)) should be"
        ~ " comparable against Object itself and use Object.opEquals.");
}

///
@system unittest
{
    static struct S
    {
        int* ptr;
    }
    S s = S(new int);

    const cs = s;
    // Can't assign s.ptr to cs.ptr
    static assert(!__traits(compiles, {s = cs;}));

    Rebindable!(const S) rs = s;
    assert(rs.ptr is s.ptr);
    // rs.ptr is const
    static assert(!__traits(compiles, {rs.ptr = null;}));

    // Can't assign s.ptr to rs.ptr
    static assert(!__traits(compiles, {s = rs;}));

    const S cs2 = rs;
    // Rebind rs
    rs = cs2;
    rs = S();
    assert(rs.ptr is null);
}

// https://issues.dlang.org/show_bug.cgi?id=18755
@safe unittest
{
    static class Foo
    {
        auto opCast(T)() @system immutable pure nothrow
        {
            *(cast(uint*) 0xdeadbeef) = 0xcafebabe;
            return T.init;
        }
    }

    static assert(!__traits(compiles, () @safe {
        auto r = Rebindable!(immutable Foo)(new Foo);
    }));
    static assert(__traits(compiles, () @system {
        auto r = Rebindable!(immutable Foo)(new Foo);
    }));
}

@safe unittest
{
    class CustomToHash
    {
        override size_t toHash() const nothrow @trusted { return 42; }
    }
    Rebindable!(immutable(CustomToHash)) a = new immutable CustomToHash();
    assert(a.toHash() == 42, "Rebindable!A should offer toHash()"
        ~ " by forwarding to A.toHash().");
}

// Test Rebindable!immutable
@safe unittest
{
    static struct S
    {
        int* ptr;
    }
    S s = S(new int);

    Rebindable!(immutable S) ri = S(new int);
    assert(ri.ptr !is null);
    static assert(!__traits(compiles, {ri.ptr = null;}));

    // ri is not compatible with mutable S
    static assert(!__traits(compiles, {s = ri;}));
    static assert(!__traits(compiles, {ri = s;}));

    auto ri2 = ri;
    assert(ri2.ptr == ri.ptr);

    const S cs3 = ri;
    static assert(!__traits(compiles, {ri = cs3;}));

    immutable S si = ri;
    // Rebind ri
    ri = si;
    ri = S();
    assert(ri.ptr is null);

    // Test RB!immutable -> RB!const
    Rebindable!(const S) rc = ri;
    assert(rc.ptr is null);
    ri = S(new int);
    rc = ri;
    assert(rc.ptr !is null);

    // test rebindable, opAssign
    rc.destroy;
    assert(rc.ptr is null);
    rc = rebindable(cs3);
    rc = rebindable(si);
    assert(rc.ptr !is null);

    ri.destroy;
    assert(ri.ptr is null);
    ri = rebindable(si);
    assert(ri.ptr !is null);
}

// Test disabled default ctor
@safe unittest
{
    static struct ND
    {
        int i;
        @disable this();
        this(int i) inout {this.i = i;}
    }
    static assert(!__traits(compiles, Rebindable!ND()));

    Rebindable!(const ND) rb = const ND(1);
    assert(rb.i == 1);
    rb = immutable ND(2);
    assert(rb.i == 2);
    rb = rebindable(const ND(3));
    assert(rb.i == 3);
    static assert(!__traits(compiles, rb.i++));
}

// Test copying
@safe unittest
{
    int del;
    int post;
    struct S
    {
        int* ptr;
        int level;
        this(this)
        {
            post++;
            level++;
        }
        ~this()
        {
            del++;
        }
    }

    // test ref count
    {
        Rebindable!S rc = S(new int);
    }
    assert(post == del - 1);
}

@safe unittest
{
    int del;
    int post;
    struct S
    {
        immutable int x;
        int level;
        this(this)
        {
            post++;
            level++;
        }
        ~this()
        {
            del++;
        }
    }

    // test ref count
    {
        Rebindable!S rc = S(0);
    }
    assert(post == del - 1);
}

/**
Convenience function for creating a `Rebindable` using automatic type
inference.

Params:
    obj = A reference to a value to initialize the `Rebindable` with.

Returns:
    A newly constructed `Rebindable` initialized with the given reference.
*/
Rebindable!T rebindable(T)(T obj)
if (is(T == class) || is(T == interface) || isDynamicArray!T || isAssociativeArray!T)
{
    typeof(return) ret;
    ret = obj;
    return ret;
}

///
@system unittest
{
    class C
    {
        int payload;
        this(int p) { payload = p; }
    }
    const c = new C(1);

    auto c2 = c.rebindable;
    assert(c2.payload == 1);
    // passing Rebindable to rebindable
    c2 = c2.rebindable;

    c2 = new C(2);
    assert(c2.payload == 2);

    const c3 = c2.get;
    assert(c3.payload == 2);
}

/// ditto
Rebindable!T rebindable(T)(T value)
if (!is(T == class) && !is(T == interface) && !isDynamicArray!T && !isAssociativeArray!T
    && !is(T : Rebindable!U, U))
{
    return Rebindable!T(value);
}

///
@safe unittest
{
    immutable struct S
    {
        int[] array;
    }
    auto s1 = [3].idup.rebindable;
    s1 = [4].idup.rebindable;
    assert(s1 == [4]);
}

/**
This function simply returns the `Rebindable` object passed in.  It's useful
in generic programming cases when a given object may be either a regular
`class` or a `Rebindable`.

Params:
    obj = An instance of Rebindable!T.

Returns:
    `obj` without any modification.
*/
Rebindable!T rebindable(T)(Rebindable!T obj)
{
    return obj;
}

// TODO: remove me once the rebindable overloads have been joined
///
@system unittest
{
    class C
    {
        int payload;
        this(int p) { payload = p; }
    }
    const c = new C(1);

    auto c2 = c.rebindable;
    assert(c2.payload == 1);
    // passing Rebindable to rebindable
    c2 = c2.rebindable;
    assert(c2.payload == 1);
}

@system unittest
{
    interface CI { int foo() const; }
    class C : CI {
      int foo() const { return 42; }
      @property int bar() const { return 23; }
    }
    Rebindable!(C) obj0;
    static assert(is(typeof(obj0) == C));

    Rebindable!(const(C)) obj1;
    static assert(is(typeof(obj1.get) == const(C)), typeof(obj1.get).stringof);
    static assert(is(typeof(obj1.stripped) == C));
    obj1 = new C;
    assert(obj1.get !is null);
    obj1 = new const(C);
    assert(obj1.get !is null);

    Rebindable!(immutable(C)) obj2;
    static assert(is(typeof(obj2.get) == immutable(C)));
    static assert(is(typeof(obj2.stripped) == C));
    obj2 = new immutable(C);
    assert(obj1.get !is null);

    // test opDot
    assert(obj2.foo() == 42);
    assert(obj2.bar == 23);

    interface I { final int foo() const { return 42; } }
    Rebindable!(I) obj3;
    static assert(is(typeof(obj3) == I));

    Rebindable!(const I) obj4;
    static assert(is(typeof(obj4.get) == const I));
    static assert(is(typeof(obj4.stripped) == I));
    static assert(is(typeof(obj4.foo()) == int));
    obj4 = new class I {};

    Rebindable!(immutable C) obj5i;
    Rebindable!(const C) obj5c;
    obj5c = obj5c;
    obj5c = obj5i;
    obj5i = obj5i;
    static assert(!__traits(compiles, obj5i = obj5c));

    // Test the convenience functions.
    auto obj5convenience = rebindable(obj5i);
    assert(obj5convenience is obj5i);

    auto obj6 = rebindable(new immutable(C));
    static assert(is(typeof(obj6) == Rebindable!(immutable C)));
    assert(obj6.foo() == 42);

    auto obj7 = rebindable(new C);
    CI interface1 = obj7;
    auto interfaceRebind1 = rebindable(interface1);
    assert(interfaceRebind1.foo() == 42);

    const interface2 = interface1;
    auto interfaceRebind2 = rebindable(interface2);
    assert(interfaceRebind2.foo() == 42);

    auto arr = [1,2,3,4,5];
    const arrConst = arr;
    assert(rebindable(arr) == arr);
    assert(rebindable(arrConst) == arr);

    // https://issues.dlang.org/show_bug.cgi?id=7654
    immutable(char[]) s7654;
    Rebindable!(typeof(s7654)) r7654 = s7654;

    static foreach (T; AliasSeq!(char, wchar, char, int))
    {
        static assert(is(Rebindable!(immutable(T[])) == immutable(T)[]));
        static assert(is(Rebindable!(const(T[])) == const(T)[]));
        static assert(is(Rebindable!(T[]) == T[]));
    }

    // Pull request 3341
    Rebindable!(immutable int[int]) pr3341 = [123:345];
    assert(pr3341[123] == 345);
    immutable int[int] pr3341_aa = [321:543];
    pr3341 = pr3341_aa;
    assert(pr3341[321] == 543);
    assert(rebindable(pr3341_aa)[321] == 543);
}

package(std) struct Rebindable2(T)
{
private:
    static if (isAssignable!(typeof(cast() T.init)))
    {
        enum useQualifierCast = true;

        typeof(cast() T.init) data;
    }
    else
    {
        enum useQualifierCast = false;

        align(T.alignof)
        static struct Payload
        {
            static if (hasIndirections!T)
            {
                void[T.sizeof] data;
            }
            else
            {
                ubyte[T.sizeof] data;
            }
        }

        Payload data;
    }

public:

    static if (!__traits(compiles, { T value; }))
    {
        @disable this();
    }

    /**
     * Constructs a `Rebindable2` from a given value.
     */
    this(T value) @trusted
    {
        static if (useQualifierCast)
        {
            this.data = cast() value;
        }
        else
        {
            set(value);
        }
    }

    /**
     * Overwrites the currently stored value with `value`.
     */
    void opAssign(this This)(T value) @trusted
    {
        clear;
        set(value);
    }

    /**
     * Returns the value currently stored in the `Rebindable2`.
     */
    T get(this This)() @property @trusted
    {
        static if (useQualifierCast)
        {
            return cast(T) this.data;
        }
        else
        {
            return *cast(T*) &this.data;
        }
    }

    /// Ditto
    inout(T) get() inout @property @trusted
    {
        static if (useQualifierCast)
        {
            return cast(inout(T)) this.data;
        }
        else
        {
            return *cast(inout(T)*) &this.data;
        }
    }

    static if (!useQualifierCast)
    {
        ~this() @trusted
        {
            clear;
        }
    }

private:

    void set(this This)(T value)
    {
        static if (useQualifierCast)
        {
            static if (hasElaborateAssign!T)
            {
                import core.lifetime : copyEmplace;
                copyEmplace(cast() value, this.data);
            }
            else
                this.data = cast() value;
        }
        else
        {
            import core.lifetime : copyEmplace;
            copyEmplace(cast() value, cast() *cast(T*) &this.data);
        }
    }

    void clear(this This)()
    {
        // work around reinterpreting cast being impossible in CTFE
        if (__ctfe)
        {
            return;
        }

        // call possible struct destructors
        static if (is(T == struct))
        {
            .destroy!(No.initialize)(*cast(T*) &this.data);
        }
    }
}

package(std) Rebindable2!T rebindable2(T)(T value)
{
    return Rebindable2!T(value);
}

// Verify that the destructor is called properly if there is one.
@system unittest
{
    {
        bool destroyed;

        struct S
        {
            int i;

            this(int i) @safe
            {
                this.i = i;
            }

            ~this() @safe
            {
                destroyed = true;
            }
        }

        {
            auto foo = rebindable2(S(42));

            // Whether destruction has occurred here depends on whether the
            // temporary gets moved or not, so we won't assume that it has or
            // hasn't happened. What we care about here is that foo gets destroyed
            // properly when it leaves the scope.
            destroyed = false;
        }
        assert(destroyed);

        {
            auto foo = rebindable2(const S(42));
            destroyed = false;
        }
        assert(destroyed);
    }

    // Test for double destruction with qualifer cast being used
    {
        static struct S
        {
            int i;
            bool destroyed;

            this(int i) @safe
            {
                this.i = i;
            }

            ~this() @safe
            {
                destroyed = true;
            }

            @safe invariant
            {
                assert(!destroyed);
            }
        }

        {
            auto foo = rebindable2(S(42));
            assert(typeof(foo).useQualifierCast);
            assert(foo.data.i == 42);
            assert(!foo.data.destroyed);
        }
        {
            auto foo = rebindable2(S(42));
            destroy(foo);
        }
        {
            auto foo = rebindable2(const S(42));
            assert(typeof(foo).useQualifierCast);
            assert(foo.data.i == 42);
            assert(!foo.data.destroyed);
        }
        {
            auto foo = rebindable2(const S(42));
            destroy(foo);
        }
    }

    // Test for double destruction without qualifer cast being used
    {
        static struct S
        {
            int i;
            bool destroyed;

            this(int i) @safe
            {
                this.i = i;
            }

            ~this() @safe
            {
                destroyed = true;
            }

            @disable ref S opAssign()(auto ref S rhs);

            @safe invariant
            {
                assert(!destroyed);
            }
        }

        {
            auto foo = rebindable2(S(42));
            assert(!typeof(foo).useQualifierCast);
            assert((cast(S*)&(foo.data)).i == 42);
            assert(!(cast(S*)&(foo.data)).destroyed);
        }
        {
            auto foo = rebindable2(S(42));
            destroy(foo);
        }
    }
}

// Verify that if there is an overloaded assignment operator, it's not assigned
// to garbage.
@safe unittest
{
    static struct S
    {
        int i;
        bool destroyed;

        this(int i) @safe
        {
            this.i = i;
        }

        ~this() @safe
        {
            destroyed = true;
        }

        ref opAssign()(auto ref S rhs)
        {
            assert(!this.destroyed);
            this.i = rhs.i;
            return this;
        }
    }

    {
        auto foo = rebindable2(S(42));
        foo = S(99);
        assert(foo.data.i == 99);
    }
    {
        auto foo = rebindable2(S(42));
        foo = const S(99);
        assert(foo.data.i == 99);
    }
}

// Verify that postblit or copy constructor is called properly if there is one.
@system unittest
{
    // postblit with type qualifier cast
    {
        static struct S
        {
            int i;
            static bool copied;

            this(this) @safe
            {
                copied = true;
            }
        }

        {
            auto foo = rebindable2(S(42));

            // Whether a copy has occurred here depends on whether the
            // temporary gets moved or not, so we won't assume that it has or
            // hasn't happened. What we care about here is that foo gets copied
            // properly when we copy it below.
            S.copied = false;

            auto bar = foo;
            assert(S.copied);
        }
        {
            auto foo = rebindable2(const S(42));
            assert(typeof(foo).useQualifierCast);
            S.copied = false;

            auto bar = foo;
            assert(S.copied);
        }
    }

    // copy constructor with type qualifier cast
    {
        static struct S
        {
            int i;
            static bool copied;

            this(ref inout S rhs) @safe inout
            {
                this.i = rhs.i;
                copied = true;
            }
        }

        {
            auto foo = rebindable2(S(42));
            assert(typeof(foo).useQualifierCast);
            S.copied = false;

            auto bar = foo;
            assert(S.copied);
        }
        {
            auto foo = rebindable2(const S(42));
            S.copied = false;

            auto bar = foo;
            assert(S.copied);
        }
    }

    // FIXME https://issues.dlang.org/show_bug.cgi?id=24829

    // Making this work requires either reworking how the !useQualiferCast
    // version works so that the compiler can correctly generate postblit
    // constructors and copy constructors as appropriate, or an explicit
    // postblit or copy constructor needs to be added for such cases, which
    // gets pretty complicated if we want to correctly add the same attributes
    // that T's postblit or copy constructor has.

    /+
    // postblit without type qualifier cast
    {
        static struct S
        {
            int* ptr;
            static bool copied;

            this(int i)
            {
                ptr = new int(i);
            }

            this(this) @safe
            {
                if (ptr !is null)
                    ptr = new int(*ptr);
                copied = true;
            }

            @disable ref S opAssign()(auto ref S rhs);
        }

        {
            auto foo = rebindable2(S(42));
            assert(!typeof(foo).useQualifierCast);
            S.copied = false;

            auto bar = foo;
            assert(S.copied);
            assert(*(cast(S*)&(foo.data)).ptr == *(cast(S*)&(bar.data)).ptr);
            assert((cast(S*)&(foo.data)).ptr !is (cast(S*)&(bar.data)).ptr);
        }
        {
            auto foo = rebindable2(const S(42));
            S.copied = false;

            auto bar = foo;
            assert(S.copied);
            assert(*(cast(S*)&(foo.data)).ptr == *(cast(S*)&(bar.data)).ptr);
            assert((cast(S*)&(foo.data)).ptr !is (cast(S*)&(bar.data)).ptr);
        }
    }

    // copy constructor without type qualifier cast
    {
        static struct S
        {
            int* ptr;
            static bool copied;

            this(int i)
            {
                ptr = new int(i);
            }

            this(ref inout S rhs) @safe inout
            {
                if (rhs.ptr !is null)
                    ptr = new inout int(*rhs.ptr);
                copied = true;
            }

            @disable ref S opAssign()(auto ref S rhs);
        }

        {
            auto foo = rebindable2(S(42));
            assert(!typeof(foo).useQualifierCast);
            S.copied = false;

            auto bar = foo;
            assert(S.copied);
            assert(*(cast(S*)&(foo.data)).ptr == *(cast(S*)&(bar.data)).ptr);
            assert((cast(S*)&(foo.data)).ptr !is (cast(S*)&(bar.data)).ptr);
        }
        {
            auto foo = rebindable2(const S(42));
            S.copied = false;

            auto bar = foo;
            assert(S.copied);
            assert(*(cast(S*)&(foo.data)).ptr == *(cast(S*)&(bar.data)).ptr);
            assert((cast(S*)&(foo.data)).ptr !is (cast(S*)&(bar.data)).ptr);
        }
    }
    +/
}

/**
    Similar to `Rebindable!(T)` but strips all qualifiers from the reference as
    opposed to just constness / immutability. Primary intended use case is with
    shared (having thread-local reference to shared class data)

    Params:
        T = A class or interface type.
 */
template UnqualRef(T)
if (is(T == class) || is(T == interface))
{
    static if (is(T == immutable U, U)
        || is(T == const shared U, U)
        || is(T == const U, U)
        || is(T == shared U, U))
    {
        struct UnqualRef
        {
            mixin RebindableCommon!(T, U, UnqualRef);
        }
    }
    else
    {
        alias UnqualRef = T;
    }
}

///
@system unittest
{
    class Data {}

    static shared(Data) a;
    static UnqualRef!(shared Data) b;

    import core.thread;

    auto thread = new core.thread.Thread({
        a = new shared Data();
        b = new shared Data();
    });

    thread.start();
    thread.join();

    assert(a !is null);
    assert(b is null);
}

@safe unittest
{
    class C { }
    alias T = UnqualRef!(const shared C);
    static assert(is(typeof(T.stripped) == C));
}



/**
  Order the provided members to minimize size while preserving alignment.
  Alignment is not always optimal for 80-bit reals, nor for structs declared
  as align(1).

  Params:
      E = A list of the types to be aligned, representing fields
          of an aggregate such as a `struct` or `class`.

      names = The names of the fields that are to be aligned.

  Returns:
      A string to be mixed in to an aggregate, such as a `struct` or `class`.
*/
string alignForSize(E...)(const char[][] names...)
{
    // Sort all of the members by .alignof.
    // BUG: Alignment is not always optimal for align(1) structs
    // or 80-bit reals or 64-bit primitives on x86.
    // TRICK: Use the fact that .alignof is always a power of 2,
    // and maximum 16 on extant systems. Thus, we can perform
    // a very limited radix sort.
    // Contains the members with .alignof = 64,32,16,8,4,2,1

    assert(E.length == names.length,
        "alignForSize: There should be as many member names as the types");

    string[7] declaration = ["", "", "", "", "", "", ""];

    foreach (i, T; E)
    {
        auto a = T.alignof;
        auto k = a >= 64? 0 : a >= 32? 1 : a >= 16? 2 : a >= 8? 3 : a >= 4? 4 : a >= 2? 5 : 6;
        declaration[k] ~= T.stringof ~ " " ~ names[i] ~ ";\n";
    }

    auto s = "";
    foreach (decl; declaration)
        s ~= decl;
    return s;
}

///
@safe unittest
{
    struct Banner {
        mixin(alignForSize!(byte[6], double)(["name", "height"]));
    }
}

@safe unittest
{
    enum x = alignForSize!(int[], char[3], short, double[5])("x", "y","z", "w");
    struct Foo { int x; }
    enum y = alignForSize!(ubyte, Foo, double)("x", "y", "z");

    enum passNormalX = x == "double[5] w;\nint[] x;\nshort z;\nchar[3] y;\n";
    enum passNormalY = y == "double z;\nFoo y;\nubyte x;\n";

    enum passAbnormalX = x == "int[] x;\ndouble[5] w;\nshort z;\nchar[3] y;\n";
    enum passAbnormalY = y == "Foo y;\ndouble z;\nubyte x;\n";
    // ^ blame https://issues.dlang.org/show_bug.cgi?id=231

    static assert(passNormalX || passAbnormalX && double.alignof <= (int[]).alignof);
    static assert(passNormalY || passAbnormalY && double.alignof <= int.alignof);
}

// https://issues.dlang.org/show_bug.cgi?id=12914
@safe unittest
{
    immutable string[] fieldNames = ["x", "y"];
    struct S
    {
        mixin(alignForSize!(byte, int)(fieldNames));
    }
}

/**
Defines a value paired with a distinctive "null" state that denotes
the absence of a value. If default constructed, a $(D
Nullable!T) object starts in the null state. Assigning it renders it
non-null. Calling `nullify` can nullify it again.

Practically `Nullable!T` stores a `T` and a `bool`.

See also:
    $(LREF apply), an alternative way to use the payload.
 */
struct Nullable(T)
{
    private union DontCallDestructorT
    {
        import std.traits : hasIndirections;
        static if (hasIndirections!T)
            T payload;
        else
            T payload = void;
    }

    private DontCallDestructorT _value = DontCallDestructorT.init;

    private bool _isNull = true;

    /**
     * Constructor initializing `this` with `value`.
     *
     * Params:
     *     value = The value to initialize this `Nullable` with.
     */
    static if (isCopyable!T)
        this(inout T value) inout
        {
            _value.payload = value;
            _isNull = false;
        }
    else
        this(T value) inout
        {
            import std.algorithm.mutation : move;
            _value.payload = move(value);
            _isNull = false;
        }

    static if (hasElaborateDestructor!T)
    {
        ~this()
        {
            if (!_isNull)
            {
                import std.traits : Unqual;
                auto ptr = () @trusted { return cast(Unqual!T*) &_value.payload; }();
                destroy!false(*ptr);
            }
        }
    }

    static if (!isCopyable!T)
        @disable this(this);
    else
    static if (__traits(hasPostblit, T))
    {
        this(this)
        {
            if (!_isNull)
                _value.payload.__xpostblit();
        }
    }
    else static if (__traits(hasCopyConstructor, T))
    {
        this(ref return scope inout Nullable!T rhs) inout
        {
            _isNull = rhs._isNull;
            if (!_isNull)
                _value.payload = rhs._value.payload;
            else
                _value = DontCallDestructorT.init;
        }
    }

    /**
     * If they are both null, then they are equal. If one is null and the other
     * is not, then they are not equal. If they are both non-null, then they are
     * equal if their values are equal.
     */
    bool opEquals(this This, Rhs)(auto ref Rhs rhs)
    if (!is(CommonType!(This, Rhs) == void))
    {
        static if (is(This == Rhs))
        {
            if (_isNull)
                return rhs._isNull;
            if (rhs._isNull)
                return false;
            return _value.payload == rhs._value.payload;
        }
        else
        {
            alias Common = CommonType!(This, Rhs);
            return cast(Common) this == cast(Common) rhs;
        }
    }

    /// Ditto
    bool opEquals(this This, Rhs)(auto ref Rhs rhs)
    if (is(CommonType!(This, Rhs) == void) && is(typeof(this.get == rhs)))
    {
        return _isNull ? false : rhs == _value.payload;
    }

    ///
    @safe unittest
    {
        Nullable!int empty;
        Nullable!int a = 42;
        Nullable!int b = 42;
        Nullable!int c = 27;

        assert(empty == empty);
        assert(empty == Nullable!int.init);
        assert(empty != a);
        assert(empty != b);
        assert(empty != c);

        assert(a == b);
        assert(a != c);

        assert(empty != 42);
        assert(a == 42);
        assert(c != 42);
    }

    @safe unittest
    {
        // Test constness
        immutable Nullable!int a = 42;
        Nullable!int b = 42;
        immutable Nullable!int c = 29;
        Nullable!int d = 29;
        immutable e = 42;
        int f = 29;
        assert(a == a);
        assert(a == b);
        assert(a != c);
        assert(a != d);
        assert(a == e);
        assert(a != f);

        // Test rvalue
        assert(a == const Nullable!int(42));
        assert(a != Nullable!int(29));
    }

    // https://issues.dlang.org/show_bug.cgi?id=17482
    @system unittest
    {
        import std.variant : Variant;
        Nullable!Variant a = Variant(12);
        assert(a == 12);
        Nullable!Variant e;
        assert(e != 12);
    }

    size_t toHash() const @safe nothrow
    {
        static if (__traits(compiles, .hashOf(_value.payload)))
            return _isNull ? 0 : .hashOf(_value.payload);
        else
            // Workaround for when .hashOf is not both @safe and nothrow.
            return _isNull ? 0 : typeid(T).getHash(&_value.payload);
    }

    /**
     * Gives the string `"Nullable.null"` if `isNull` is `true`. Otherwise, the
     * result is equivalent to calling $(REF formattedWrite, std,format) on the
     * underlying value.
     *
     * Params:
     *     writer = A `char` accepting
     *     $(REF_ALTTEXT output range, isOutputRange, std, range, primitives)
     *     fmt = A $(REF FormatSpec, std,format) which is used to represent
     *     the value if this Nullable is not null
     * Returns:
     *     A `string` if `writer` and `fmt` are not set; `void` otherwise.
     */
    string toString()
    {
        import std.array : appender;
        auto app = appender!string();
        auto spec = singleSpec("%s");
        toString(app, spec);
        return app.data;
    }

    /// ditto
    string toString() const
    {
        import std.array : appender;
        auto app = appender!string();
        auto spec = singleSpec("%s");
        toString(app, spec);
        return app.data;
    }

    /// ditto
    void toString(W)(ref W writer, scope const ref FormatSpec!char fmt)
    if (isOutputRange!(W, char))
    {
        import std.range.primitives : put;
        if (isNull)
            put(writer, "Nullable.null");
        else
            formatValue(writer, _value.payload, fmt);
    }

    /// ditto
    void toString(W)(ref W writer, scope const ref FormatSpec!char fmt) const
    if (isOutputRange!(W, char))
    {
        import std.range.primitives : put;
        if (isNull)
            put(writer, "Nullable.null");
        else
            formatValue(writer, _value.payload, fmt);
    }

    /**
     * Check if `this` is in the null state.
     *
     * Returns:
     *     true $(B iff) `this` is in the null state, otherwise false.
     */
    @property bool isNull() const @safe pure nothrow
    {
        return _isNull;
    }

    ///
    @safe unittest
    {
        Nullable!int ni;
        assert(ni.isNull);

        ni = 0;
        assert(!ni.isNull);
    }

    // https://issues.dlang.org/show_bug.cgi?id=14940
    @safe unittest
    {
        import std.array : appender;
        import std.format.write : formattedWrite;

        auto app = appender!string();
        Nullable!int a = 1;
        formattedWrite(app, "%s", a);
        assert(app.data == "1");
    }

    // https://issues.dlang.org/show_bug.cgi?id=19799
    @safe unittest
    {
        import std.format : format;

        const Nullable!string a = const(Nullable!string)();

        format!"%s"(a);
    }

    /**
     * Returns true if `this` has a value, otherwise false.
     *
     * Allows a `Nullable` to be used as the condition in an `if` statement:
     *
     * ---
     * if (auto result = functionReturningNullable())
     * {
     *     doSomethingWith(result.get);
     * }
     * ---
     */
    bool opCast(T : bool)() const
    {
        return !isNull;
    }

    /// Prevents `opCast` from disabling built-in conversions.
    auto ref T opCast(T, this This)()
    if (is(This : T) || This.sizeof == T.sizeof)
    {
        static if (is(This : T))
            // Convert implicitly
            return this;
        else
            // Reinterpret
            return *cast(T*) &this;
    }

    /**
     * Forces `this` to the null state.
     */
    void nullify()()
    {
        static if (is(T == class) || is(T == interface))
            _value.payload = null;
        else
            .destroy(_value.payload);
        _isNull = true;
    }

    ///
    @safe unittest
    {
        Nullable!int ni = 0;
        assert(!ni.isNull);

        ni.nullify();
        assert(ni.isNull);
    }

    /**
     * Assigns `value` to the internally-held state. If the assignment
     * succeeds, `this` becomes non-null.
     *
     * Params:
     *     value = A value of type `T` to assign to this `Nullable`.
     */
    ref Nullable opAssign()(T value) return
    {
        import std.algorithm.mutation : moveEmplace, move;

        if (_isNull)
        {
            // trusted since payload is known to be uninitialized.
            () @trusted { moveEmplace(value, _value.payload); }();
        }
        else
        {
            move(value, _value.payload);
        }
        _isNull = false;
        return this;
    }

    /**
     * If this `Nullable` wraps a type that already has a null value
     * (such as a pointer), then assigning the null value to this
     * `Nullable` is no different than assigning any other value of
     * type `T`, and the resulting code will look very strange. It
     * is strongly recommended that this be avoided by instead using
     * the version of `Nullable` that takes an additional `nullValue`
     * template argument.
     */
    @safe unittest
    {
        //Passes
        Nullable!(int*) npi;
        assert(npi.isNull);

        //Passes?!
        npi = null;
        assert(!npi.isNull);
    }

    /**
     * Gets the value if not null. If `this` is in the null state, and the optional
     * parameter `fallback` was provided, it will be returned. Without `fallback`,
     * calling `get` with a null state is invalid.
     *
     * When the fallback type is different from the Nullable type, `get(T)` returns
     * the common type.
     *
     * Params:
     *     fallback = the value to return in case the `Nullable` is null.
     *
     * Returns:
     *     The value held internally by this `Nullable`.
     */
    @property ref inout(T) get() inout @safe pure nothrow
    {
        enum message = "Called `get' on null Nullable!" ~ T.stringof ~ ".";
        assert(!isNull, message);
        return _value.payload;
    }

    /// ditto
    @property inout(T) get()(inout(T) fallback) inout
    {
        return isNull ? fallback : _value.payload;
    }

    /// ditto
    @property auto get(U)(inout(U) fallback) inout
    {
        return isNull ? fallback : _value.payload;
    }

    /// $(MREF_ALTTEXT Range interface, std, range, primitives) functions.
    alias empty = isNull;

    /// ditto
    alias popFront = nullify;

    /// ditto
    alias popBack = nullify;

    /// ditto
    @property ref inout(T) front() inout @safe pure nothrow
    {
        return get();
    }

    /// ditto
    alias back = front;

    /// ditto
    static if (isCopyable!T)
    @property inout(typeof(this)) save() inout
    {
        return this;
    }

    /// ditto
    static if (isCopyable!T)
    inout(typeof(this)) opIndex(size_t[2] dim) inout
    in (dim[0] <= length && dim[1] <= length && dim[1] >= dim[0])
    {
        return (dim[0] == 0 && dim[1] == 1) ? this : this.init;
    }
    /// ditto
    size_t[2] opSlice(size_t dim : 0)(size_t from, size_t to) const
    {
        return [from, to];
    }

    /// ditto
    @property size_t length() const @safe pure nothrow
    {
        return !empty;
    }

    /// ditto
    alias opDollar(size_t dim : 0) = length;

    /// ditto
    ref inout(T) opIndex(size_t index) inout @safe pure nothrow
    in (index < length)
    {
        return get();
    }

    /**
     * Converts `Nullable` to a range. Works even when the contained type is `immutable`.
     */
    auto opSlice(this This)()
    {
        static struct NullableRange
        {
            private This value;

            // starts out true if value is null
            private bool empty_;

            @property bool empty() const @safe pure nothrow
            {
                return empty_;
            }

            void popFront() @safe pure nothrow
            {
                empty_ = true;
            }

            alias popBack = popFront;

            @property ref inout(typeof(value.get())) front() inout @safe pure nothrow
            {
                return value.get();
            }

            alias back = front;

            @property inout(typeof(this)) save() inout
            {
                return this;
            }

            size_t[2] opSlice(size_t dim : 0)(size_t from, size_t to) const
            {
                return [from, to];
            }

            @property size_t length() const @safe pure nothrow
            {
                return !empty;
            }

            alias opDollar(size_t dim : 0) = length;

            ref inout(typeof(value.get())) opIndex(size_t index) inout @safe pure nothrow
            in (index < length)
            {
                return value.get();
            }

            inout(typeof(this)) opIndex(size_t[2] dim) inout
            in (dim[0] <= length && dim[1] <= length && dim[1] >= dim[0])
            {
                return (dim[0] == 0 && dim[1] == 1) ? this : this.init;
            }

            auto opIndex() inout
            {
                return this;
            }
        }
        return NullableRange(this, isNull);
    }
}

/// ditto
auto nullable(T)(T t)
{
    return Nullable!T(t);
}

///
@safe unittest
{
    struct CustomerRecord
    {
        string name;
        string address;
        int customerNum;
    }

    Nullable!CustomerRecord getByName(string name)
    {
        //A bunch of hairy stuff

        return Nullable!CustomerRecord.init;
    }

    auto queryResult = getByName("Doe, John");
    if (!queryResult.isNull)
    {
        //Process Mr. Doe's customer record
        auto address = queryResult.get.address;
        auto customerNum = queryResult.get.customerNum;

        //Do some things with this customer's info
    }
    else
    {
        //Add the customer to the database
    }
}

///
@system unittest
{
    import std.exception : assertThrown;

    auto a = 42.nullable;
    assert(!a.isNull);
    assert(a.get == 42);

    a.nullify();
    assert(a.isNull);
    assertThrown!Throwable(a.get);
}
///
@safe unittest
{
    import std.algorithm.iteration : each, joiner;
    Nullable!int a = 42;
    Nullable!int b;
    // Add each value to an array
    int[] arr;
    a.each!((n) => arr ~= n);
    assert(arr == [42]);
    b.each!((n) => arr ~= n);
    assert(arr == [42]);
    // Take first value from an array of Nullables
    Nullable!int[] c = new Nullable!int[](10);
    c[7] = Nullable!int(42);
    assert(c.joiner.front == 42);
}
@safe unittest
{
    auto k = Nullable!int(74);
    assert(k == 74);
    k.nullify();
    assert(k.isNull);
}
@safe unittest
{
    static int f(scope const Nullable!int x) {
        return x.isNull ? 42 : x.get;
    }
    Nullable!int a;
    assert(f(a) == 42);
    a = 8;
    assert(f(a) == 8);
    a.nullify();
    assert(f(a) == 42);
}
@system unittest
{
    import std.exception : assertThrown;

    static struct S { int x; }
    Nullable!S s;
    assert(s.isNull);
    s = S(6);
    assert(s == S(6));
    assert(s != S(0));
    assert(s.get != S(0));
    s.get.x = 9190;
    assert(s.get.x == 9190);
    s.nullify();
    assertThrown!Throwable(s.get.x = 9441);
}
@safe unittest
{
    // Ensure Nullable can be used in pure/nothrow/@safe environment.
    function() @safe pure nothrow
    {
        Nullable!int n;
        assert(n.isNull);
        n = 4;
        assert(!n.isNull);
        assert(n == 4);
        n.nullify();
        assert(n.isNull);
    }();
}
@system unittest
{
    // Ensure Nullable can be used when the value is not pure/nothrow/@safe
    static struct S
    {
        int x;
        this(this) @system {}
    }

    Nullable!S s;
    assert(s.isNull);
    s = S(5);
    assert(!s.isNull);
    assert(s.get.x == 5);
    s.nullify();
    assert(s.isNull);
}

// https://issues.dlang.org/show_bug.cgi?id=9404
@safe unittest
{
    alias N = Nullable!int;

    void foo(N a)
    {
        N b;
        b = a; // `N b = a;` works fine
    }
    N n;
    foo(n);
}
@safe unittest
{
    //Check nullable immutable is constructable
    {
        auto a1 = Nullable!(immutable int)();
        auto a2 = Nullable!(immutable int)(1);
        auto i = a2.get;
    }
    //Check immutable nullable is constructable
    {
        auto a1 = immutable (Nullable!int)();
        auto a2 = immutable (Nullable!int)(1);
        auto i = a2.get;
    }
}
@safe unittest
{
    alias NInt   = Nullable!int;

    //Construct tests
    {
        //from other Nullable null
        NInt a1;
        NInt b1 = a1;
        assert(b1.isNull);

        //from other Nullable non-null
        NInt a2 = NInt(1);
        NInt b2 = a2;
        assert(b2 == 1);

        //Construct from similar nullable
        auto a3 = immutable(NInt)();
        NInt b3 = a3;
        assert(b3.isNull);
    }

    //Assign tests
    {
        //from other Nullable null
        NInt a1;
        NInt b1;
        b1 = a1;
        assert(b1.isNull);

        //from other Nullable non-null
        NInt a2 = NInt(1);
        NInt b2;
        b2 = a2;
        assert(b2 == 1);

        //Construct from similar nullable
        auto a3 = immutable(NInt)();
        NInt b3 = a3;
        b3 = a3;
        assert(b3.isNull);
    }
}
@safe unittest
{
    //Check nullable is nicelly embedable in a struct
    static struct S1
    {
        Nullable!int ni;
    }
    static struct S2 //inspired from 9404
    {
        Nullable!int ni;
        this(ref S2 other)
        {
            ni = other.ni;
        }
        void opAssign(ref S2 other)
        {
            ni = other.ni;
        }
    }
    static foreach (S; AliasSeq!(S1, S2))
    {{
        S a;
        S b = a;
        S c;
        c = a;
    }}
}

// https://issues.dlang.org/show_bug.cgi?id=10268
@system unittest
{
    import std.json;
    JSONValue value = null;
    auto na = Nullable!JSONValue(value);

    struct S1 { int val; }
    struct S2 { int* val; }
    struct S3 { immutable int* val; }

    {
        auto sm = S1(1);
        immutable si = immutable S1(1);
        auto x1 =           Nullable!S1(sm);
        auto x2 = immutable Nullable!S1(sm);
        auto x3 =           Nullable!S1(si);
        auto x4 = immutable Nullable!S1(si);
        assert(x1.get.val == 1);
        assert(x2.get.val == 1);
        assert(x3.get.val == 1);
        assert(x4.get.val == 1);
    }

    auto nm = 10;
    immutable ni = 10;

    {
        auto sm = S2(&nm);
        immutable si = immutable S2(&ni);
        auto x1 =           Nullable!S2(sm);
        static assert(!__traits(compiles, { auto x2 = immutable Nullable!S2(sm); }));
        static assert(!__traits(compiles, { auto x3 =           Nullable!S2(si); }));
        auto x4 = immutable Nullable!S2(si);
        assert(*x1.get.val == 10);
        assert(*x4.get.val == 10);
    }

    {
        auto sm = S3(&ni);
        immutable si = immutable S3(&ni);
        auto x1 =           Nullable!S3(sm);
        auto x2 = immutable Nullable!S3(sm);
        auto x3 =           Nullable!S3(si);
        auto x4 = immutable Nullable!S3(si);
        assert(*x1.get.val == 10);
        assert(*x2.get.val == 10);
        assert(*x3.get.val == 10);
        assert(*x4.get.val == 10);
    }
}

// https://issues.dlang.org/show_bug.cgi?id=10357
@safe unittest
{
    import std.datetime;
    Nullable!SysTime time = SysTime(0);
}

// https://issues.dlang.org/show_bug.cgi?id=10915
@system unittest
{
    import std.conv : to;
    import std.array;

    Appender!string buffer;

    Nullable!int ni;
    assert(ni.to!string() == "Nullable.null");
    assert((cast(const) ni).to!string() == "Nullable.null");

    struct Test { string s; }
    alias NullableTest = Nullable!Test;

    NullableTest nt = Test("test");
    // test output range version
    assert(nt.to!string() == `Test("test")`);
    // test appender version
    assert(nt.toString() == `Test("test")`);
    // test const version
    assert((cast(const) nt).toString() == `const(Test)("test")`);

    NullableTest ntn = Test("null");
    assert(ntn.to!string() == `Test("null")`);

    class TestToString
    {
        double d;

        this (double d)
        {
            this.d = d;
        }

        override string toString()
        {
            return d.to!string();
        }
    }
    Nullable!TestToString ntts = new TestToString(2.5);
    assert(ntts.to!string() == "2.5");
}

// https://issues.dlang.org/show_bug.cgi?id=14477
@safe unittest
{
    static struct DisabledDefaultConstructor
    {
        @disable this();
        this(int i) { }
    }
    Nullable!DisabledDefaultConstructor var;
    var = DisabledDefaultConstructor(5);
    var.nullify;
}

// https://issues.dlang.org/show_bug.cgi?id=17440
@system unittest
{
    static interface I { }

    static class C : I
    {
        int canary;
        ~this()
        {
            canary = 0x5050DEAD;
        }
    }
    auto c = new C;
    c.canary = 0xA71FE;
    auto nc = nullable(c);
    nc.nullify;
    assert(c.canary == 0xA71FE);

    I i = c;
    auto ni = nullable(i);
    ni.nullify;
    assert(c.canary == 0xA71FE);
}

// https://issues.dlang.org/show_bug.cgi?id=19037
@safe unittest
{
    import std.datetime : SysTime;

    struct Test
    {
        SysTime _st;

        static bool destroyed;

        @disable this();
        this(int _dummy) {}
        ~this() @safe { destroyed = true; }

        // mustn't call opAssign on Test.init in Nullable!Test, because the invariant
        // will be called before opAssign on the Test.init that is in Nullable
        // and Test.init violates its invariant.
        void opAssign(Test rhs) @safe { assert(false); }
    }

    {
        Nullable!Test nt;

        nt = Test(1);

        // destroy value
        Test.destroyed = false;

        nt.nullify;

        assert(Test.destroyed);

        Test.destroyed = false;
    }
    // don't run destructor on T.init in Nullable on scope exit!
    assert(!Test.destroyed);
}
// check that the contained type's destructor is called on assignment
@system unittest
{
    struct S
    {
        // can't be static, since we need a specific value's pointer
        bool* destroyedRef;

        ~this()
        {
            if (this.destroyedRef)
            {
                *this.destroyedRef = true;
            }
        }
    }

    Nullable!S ns;

    bool destroyed;

    ns = S(&destroyed);

    // reset from rvalue destruction in Nullable's opAssign
    destroyed = false;

    // overwrite Nullable
    ns = S(null);

    // the original S should be destroyed.
    assert(destroyed == true);
}
// check that the contained type's destructor is still called when required
@system unittest
{
    bool destructorCalled = false;

    struct S
    {
        bool* destroyed;
        ~this() { *this.destroyed = true; }
    }

    {
        Nullable!S ns;
    }
    assert(!destructorCalled);
    {
        Nullable!S ns = Nullable!S(S(&destructorCalled));

        destructorCalled = false; // reset after S was destroyed in the NS constructor
    }
    assert(destructorCalled);
}

// check that toHash on Nullable is forwarded to the contained type
@system unittest
{
    struct S
    {
        size_t toHash() const @safe pure nothrow { return 5; }
    }

    Nullable!S s1 = S();
    Nullable!S s2 = Nullable!S();

    assert(typeid(Nullable!S).getHash(&s1) == 5);
    assert(typeid(Nullable!S).getHash(&s2) == 0);
}

// https://issues.dlang.org/show_bug.cgi?id=21704
@safe unittest
{
    import std.array : staticArray;

    bool destroyed;

    struct Probe
    {
        ~this() { destroyed = true; }
    }

    {
        Nullable!(Probe[1]) test = [Probe()].staticArray;
        destroyed = false;
    }
    assert(destroyed);
}

// https://issues.dlang.org/show_bug.cgi?id=21705
@safe unittest
{
    static struct S
    {
        int n;
        bool opEquals(S rhs) { return n == rhs.n; }
    }

    Nullable!S test1 = S(1), test2 = S(1);
    S s = S(1);

    assert(test1 == s);
    assert(test1 == test2);
}

// https://issues.dlang.org/show_bug.cgi?id=22101
@safe unittest
{
    static int impure;

    struct S
    {
        ~this() { impure++; }
    }

    Nullable!S s;
    s.get(S());
}

// https://issues.dlang.org/show_bug.cgi?id=22100
@safe unittest
{
    Nullable!int a, b, c;
    a = b = c = 5;
    a = b = c = nullable(5);
}

// https://issues.dlang.org/show_bug.cgi?id=18374
@safe pure nothrow unittest
{
    import std.algorithm.comparison : equal;
    import std.range : only, takeNone;
    import std.range.primitives : hasAssignableElements, hasLength,
        hasLvalueElements, hasSlicing, hasSwappableElements,
        isRandomAccessRange;
    Nullable!int a = 42;
    assert(!a.empty);
    assert(a.front == 42);
    assert(a.back == 42);
    assert(a[0] == 42);
    assert(a.equal(only(42)));
    assert(a[0 .. $].equal(only(42)));
    a[0] = 43;
    assert(a.equal(only(43)));
    --a[0];
    assert(a.equal(only(42)));
    Nullable!int b;
    assert(b.empty);
    assert(b.equal(takeNone(b)));
    Nullable!int c = a.save();
    assert(!c.empty);
    c.popFront();
    assert(!a.empty);
    assert(c.empty);

    assert(isRandomAccessRange!(Nullable!int));
    assert(hasLength!(Nullable!int));
    assert(hasSlicing!(Nullable!int));
    assert(hasAssignableElements!(Nullable!int));
    assert(hasSwappableElements!(Nullable!int));
    assert(hasLvalueElements!(Nullable!int));
}

// https://issues.dlang.org/show_bug.cgi?id=23640
@safe pure nothrow unittest
{
    import std.algorithm.comparison : equal;
    import std.range : only;
    import std.range.primitives : hasLength, hasSlicing,
        isRandomAccessRange;
    static immutable struct S { int[] array; }
    auto value = S([42]);
    alias ImmutableNullable = immutable Nullable!S;
    auto a = ImmutableNullable(value)[];
    alias Range = typeof(a);
    assert(isRandomAccessRange!Range);
    assert(hasLength!Range);
    assert(hasSlicing!Range);
    assert(!a.empty);
    assert(a.front == value);
    assert(a.back == value);
    assert(a[0] == value);
    assert(a.equal(only(value)));
    assert(a[0 .. $].equal(only(value)));
    Range b = a.save();
    assert(!b.empty);
    b.popFront();
    assert(!a.empty);
    assert(b.empty);
}

// https://issues.dlang.org/show_bug.cgi?id=24403
@safe unittest
{
    static bool destroyed;
    static struct S { ~this() { destroyed = true; } }

    {
        Nullable!S s = S.init;
        destroyed = false;
    }
    assert(destroyed);

    {
        Nullable!(const S) s = S.init;
        destroyed = false;
    }
    assert(destroyed);

    {
        Nullable!(immutable S) s = S.init;
        destroyed = false;
    }
    assert(destroyed);

    {
        Nullable!(shared S) s = S.init;
        destroyed = false;
    }
    assert(destroyed);
}

// https://issues.dlang.org/show_bug.cgi?id=22293
@safe unittest
{
    Nullable!int empty;
    Nullable!int full = 123;

    assert(cast(bool) empty == false);
    assert(cast(bool) full == true);

    if (empty) assert(0);
    if (!full) assert(0);
}

// check that opCast doesn't break unsafe casts
@system unittest
{
    Nullable!(const(int*)) a;
    auto result = cast(immutable(Nullable!(int*))) a;
}

/**
Just like `Nullable!T`, except that the null state is defined as a
particular value. For example, $(D Nullable!(uint, uint.max)) is an
`uint` that sets aside the value `uint.max` to denote a null
state. $(D Nullable!(T, nullValue)) is more storage-efficient than $(D
Nullable!T) because it does not need to store an extra `bool`.

Params:
    T = The wrapped type for which Nullable provides a null value.

    nullValue = The null value which denotes the null state of this
                `Nullable`. Must be of type `T`.
 */
struct Nullable(T, T nullValue)
{
    private T _value = nullValue;

/**
Constructor initializing `this` with `value`.

Params:
    value = The value to initialize this `Nullable` with.
 */
    this(T value)
    {
        _value = value;
    }

    template toString()
    {
        import std.format.spec : FormatSpec;
        import std.format.write : formatValue;
        // Needs to be a template because of https://issues.dlang.org/show_bug.cgi?id=13737.
        void toString()(scope void delegate(const(char)[]) sink, scope const ref FormatSpec!char fmt)
        {
            if (isNull)
            {
                sink.formatValue("Nullable.null", fmt);
            }
            else
            {
                sink.formatValue(_value, fmt);
            }
        }

        void toString()(scope void delegate(const(char)[]) sink, scope const ref FormatSpec!char fmt) const
        {
            if (isNull)
            {
                sink.formatValue("Nullable.null", fmt);
            }
            else
            {
                sink.formatValue(_value, fmt);
            }
        }
    }

@system unittest
{
    import std.conv : to;

    const Nullable!(ulong, 0) x = 1;
    assert(x.to!string == "1");
}

/**
Check if `this` is in the null state.

Returns:
    true $(B iff) `this` is in the null state, otherwise false.
 */
    @property bool isNull() const
    {
        //Need to use 'is' if T is a nullable type and
        //nullValue is null, or it's a compiler error
        static if (is(CommonType!(T, typeof(null)) == T) && nullValue is null)
        {
            return _value is nullValue;
        }
        //Need to use 'is' if T is a float type
        //because NaN != NaN
        else static if (__traits(isFloating, T) || __traits(compiles, { static assert(!(nullValue == nullValue)); }))
        {
            return _value is nullValue;
        }
        else
        {
            return _value == nullValue;
        }
    }

///
@safe unittest
{
    Nullable!(int, -1) ni;
    //Initialized to "null" state
    assert(ni.isNull);

    ni = 0;
    assert(!ni.isNull);
}

@system unittest
{
    assert(typeof(this).init.isNull, typeof(this).stringof ~
        ".isNull does not work correctly because " ~ T.stringof ~
        " has an == operator that is non-reflexive and could not be" ~
        " determined before runtime to be non-reflexive!");
}

// https://issues.dlang.org/show_bug.cgi?id=11135
// disable test until https://issues.dlang.org/show_bug.cgi?id=15316 gets fixed
version (none) @system unittest
{
    static foreach (T; AliasSeq!(float, double, real))
    {{
        Nullable!(T, T.init) nf;
        //Initialized to "null" state
        assert(nf.isNull);
        assert(nf is typeof(nf).init);

        nf = 0;
        assert(!nf.isNull);

        nf.nullify();
        assert(nf.isNull);
    }}
}

/**
Forces `this` to the null state.
 */
    void nullify()()
    {
        _value = nullValue;
    }

///
@safe unittest
{
    Nullable!(int, -1) ni = 0;
    assert(!ni.isNull);

    ni = -1;
    assert(ni.isNull);
}

/**
Assigns `value` to the internally-held state. If the assignment
succeeds, `this` becomes non-null. No null checks are made. Note
that the assignment may leave `this` in the null state.

Params:
    value = A value of type `T` to assign to this `Nullable`.
            If it is `nullvalue`, then the internal state of
            this `Nullable` will be set to null.
 */
    void opAssign()(T value)
    {
        import std.algorithm.mutation : swap;

        swap(value, _value);
    }

/**
    If this `Nullable` wraps a type that already has a null value
    (such as a pointer), and that null value is not given for
    `nullValue`, then assigning the null value to this `Nullable`
    is no different than assigning any other value of type `T`,
    and the resulting code will look very strange. It is strongly
    recommended that this be avoided by using `T`'s "built in"
    null value for `nullValue`.
 */
@system unittest
{
    //Passes
    enum nullVal = cast(int*) 0xCAFEBABE;
    Nullable!(int*, nullVal) npi;
    assert(npi.isNull);

    //Passes?!
    npi = null;
    assert(!npi.isNull);
}

/**
Gets the value. `this` must not be in the null state.
This function is also called for the implicit conversion to `T`.

Preconditions: `isNull` must be `false`.
Returns:
    The value held internally by this `Nullable`.
 */
    @property ref inout(T) get() inout
    {
        //@@@6169@@@: We avoid any call that might evaluate nullValue's %s,
        //Because it might messup get's purity and safety inference.
        enum message = "Called `get' on null Nullable!(" ~ T.stringof ~ ",nullValue).";
        assert(!isNull, message);
        return _value;
    }

///
@system unittest
{
    import std.exception : assertThrown, assertNotThrown;

    Nullable!(int, -1) ni;
    //`get` is implicitly called. Will throw
    //an error in non-release mode
    assertThrown!Throwable(ni == 0);

    ni = 0;
    assertNotThrown!Throwable(ni == 0);
}

/**
Implicitly converts to `T`.
`this` must not be in the null state.
 */
    alias get this;
}

/// ditto
auto nullable(alias nullValue, T)(T t)
if (is (typeof(nullValue) == T))
{
    return Nullable!(T, nullValue)(t);
}

///
@safe unittest
{
    Nullable!(size_t, size_t.max) indexOf(string[] haystack, string needle)
    {
        //Find the needle, returning -1 if not found

        return Nullable!(size_t, size_t.max).init;
    }

    void sendLunchInvite(string name)
    {
    }

    //It's safer than C...
    auto coworkers = ["Jane", "Jim", "Marry", "Fred"];
    auto pos = indexOf(coworkers, "Bob");
    if (!pos.isNull)
    {
        //Send Bob an invitation to lunch
        sendLunchInvite(coworkers[pos]);
    }
    else
    {
        //Bob not found; report the error
    }

    //And there's no overhead
    static assert(Nullable!(size_t, size_t.max).sizeof == size_t.sizeof);
}

///
@system unittest
{
    import std.exception : assertThrown;

    Nullable!(int, int.min) a;
    assert(a.isNull);
    assertThrown!Throwable(a.get);
    a = 5;
    assert(!a.isNull);
    assert(a == 5);
    static assert(a.sizeof == int.sizeof);
}

///
@safe unittest
{
    auto a = nullable!(int.min)(8);
    assert(a == 8);
    a.nullify();
    assert(a.isNull);
}

@nogc nothrow pure @safe unittest
{
    // https://issues.dlang.org/show_bug.cgi?id=19226
    // fully handle non-self-equal nullValue
    static struct Fraction
    {
        int denominator;
        bool isNaN() const
        {
            return denominator == 0;
        }
        bool opEquals(const Fraction rhs) const
        {
            return !isNaN && denominator == rhs.denominator;
        }
    }
    alias N = Nullable!(Fraction, Fraction.init);
    assert(N.init.isNull);
}

@safe unittest
{
    static int f(scope const Nullable!(int, int.min) x) {
        return x.isNull ? 42 : x.get;
    }
    Nullable!(int, int.min) a;
    assert(f(a) == 42);
    a = 8;
    assert(f(a) == 8);
    a.nullify();
    assert(f(a) == 42);
}
@safe unittest
{
    // Ensure Nullable can be used in pure/nothrow/@safe environment.
    function() @safe pure nothrow
    {
        Nullable!(int, int.min) n;
        assert(n.isNull);
        n = 4;
        assert(!n.isNull);
        assert(n == 4);
        n.nullify();
        assert(n.isNull);
    }();
}
@system unittest
{
    // Ensure Nullable can be used when the value is not pure/nothrow/@system
    static struct S
    {
        int x;
        bool opEquals(const S s) const @system { return s.x == x; }
    }

    Nullable!(S, S(711)) s;
    assert(s.isNull);
    s = S(5);
    assert(!s.isNull);
    assert(s.x == 5);
    s.nullify();
    assert(s.isNull);
}
@safe unittest
{
    //Check nullable is nicelly embedable in a struct
    static struct S1
    {
        Nullable!(int, 0) ni;
    }
    static struct S2 //inspired from 9404
    {
        Nullable!(int, 0) ni;
        this(S2 other)
        {
            ni = other.ni;
        }
        void opAssign(S2 other)
        {
            ni = other.ni;
        }
    }
    static foreach (S; AliasSeq!(S1, S2))
    {{
        S a;
        S b = a;
        S c;
        c = a;
    }}
}
@system unittest
{
    import std.conv : to;

    // https://issues.dlang.org/show_bug.cgi?id=10915
    Nullable!(int, 1) ni = 1;
    assert(ni.to!string() == "Nullable.null");

    struct Test { string s; }
    alias NullableTest = Nullable!(Test, Test("null"));

    NullableTest nt = Test("test");
    assert(nt.to!string() == `Test("test")`);

    NullableTest ntn = Test("null");
    assert(ntn.to!string() == "Nullable.null");

    class TestToString
    {
        double d;

        this(double d)
        {
            this.d = d;
        }

        override string toString()
        {
            return d.to!string();
        }
    }
    alias NullableTestToString = Nullable!(TestToString, null);

    NullableTestToString ntts = new TestToString(2.5);
    assert(ntts.to!string() == "2.5");
}

// apply
/**
Unpacks the content of a `Nullable`, performs an operation and packs it again. Does nothing if isNull.

When called on a `Nullable`, `apply` will unpack the value contained in the `Nullable`,
pass it to the function you provide and wrap the result in another `Nullable` (if necessary).
If the `Nullable` is null, `apply` will return null itself.

Params:
    t = a `Nullable`
    fun = a function operating on the content of the nullable

Returns:
    `fun(t.get).nullable` if `!t.isNull`, else `Nullable.init`.

See also:
    $(HTTPS en.wikipedia.org/wiki/Monad_(functional_programming)#The_Maybe_monad, The `Maybe` monad)
*/
template apply(alias fun)
{
    import std.functional : unaryFun;

    auto apply(T)(auto ref T t)
    if (isInstanceOf!(Nullable, T))
    {
        alias FunType = typeof(unaryFun!fun(T.init.get));

        enum MustWrapReturn = !isInstanceOf!(Nullable, FunType);

        static if (MustWrapReturn)
        {
            alias ReturnType = Nullable!FunType;
        }
        else
        {
            alias ReturnType = FunType;
        }

        if (!t.isNull)
        {
            static if (MustWrapReturn)
            {
                return unaryFun!fun(t.get).nullable;
            }
            else
            {
                return unaryFun!fun(t.get);
            }
        }
        else
        {
            return ReturnType.init;
        }
    }
}

///
nothrow pure @nogc @safe unittest
{
    alias toFloat = i => cast(float) i;

    Nullable!int sample;

    // apply(null) results in a null `Nullable` of the function's return type.
    Nullable!float f = sample.apply!toFloat;
    assert(sample.isNull && f.isNull);

    sample = 3;

    // apply(non-null) calls the function and wraps the result in a `Nullable`.
    f = sample.apply!toFloat;
    assert(!sample.isNull && !f.isNull);
    assert(f.get == 3.0f);
}

///
nothrow pure @nogc @safe unittest
{
    alias greaterThree = i => (i > 3) ? i.nullable : Nullable!(typeof(i)).init;

    Nullable!int sample;

    // when the function already returns a `Nullable`, that `Nullable` is not wrapped.
    auto result = sample.apply!greaterThree;
    assert(sample.isNull && result.isNull);

    // The function may decide to return a null `Nullable`.
    sample = 3;
    result = sample.apply!greaterThree;
    assert(!sample.isNull && result.isNull);

    // Or it may return a value already wrapped in a `Nullable`.
    sample = 4;
    result = sample.apply!greaterThree;
    assert(!sample.isNull && !result.isNull);
    assert(result.get == 4);
}

// test that Nullable.get(default) can merge types
@safe @nogc nothrow pure
unittest
{
    Nullable!ubyte sample = Nullable!ubyte();

    // Test that get(U) returns the common type of the Nullable type and the parameter type.
    assert(sample.get(1000) == 1000);
}

// Workaround for https://issues.dlang.org/show_bug.cgi?id=20670
@safe @nogc nothrow pure
unittest
{
    immutable struct S { }

    S[] array = Nullable!(S[])().get(S[].init);
}

// regression test for https://issues.dlang.org/show_bug.cgi?id=21199
@safe @nogc nothrow pure
unittest
{
    struct S { int i; }
    assert(S(5).nullable.apply!"a.i" == 5);
}

// regression test for https://issues.dlang.org/show_bug.cgi?id=22176
@safe @nogc nothrow pure
unittest
{
    struct S
    {
        int i;
        invariant(i != 0);

        // Nullable shouldn't cause S to generate an
        // opAssign that would check the invariant.
        Nullable!int j;
    }
    S s;
    s = S(5);
}

/**
Just like `Nullable!T`, except that the object refers to a value
sitting elsewhere in memory. This makes assignments overwrite the
initially assigned value. Internally `NullableRef!T` only stores a
pointer to `T` (i.e., $(D Nullable!T.sizeof == (T*).sizeof)).
 */
struct NullableRef(T)
{
    private T* _value;

/**
Constructor binding `this` to `value`.

Params:
    value = The value to bind to.
 */
    this(T* value) @safe pure nothrow
    {
        _value = value;
    }

    template toString()
    {
        import std.format.spec : FormatSpec;
        import std.format.write : formatValue;
        // Needs to be a template because of https://issues.dlang.org/show_bug.cgi?id=13737.
        void toString()(scope void delegate(const(char)[]) sink, scope const ref FormatSpec!char fmt)
        {
            if (isNull)
            {
                sink.formatValue("Nullable.null", fmt);
            }
            else
            {
                sink.formatValue(*_value, fmt);
            }
        }

        void toString()(scope void delegate(const(char)[]) sink, scope const ref FormatSpec!char fmt) const
        {
            if (isNull)
            {
                sink.formatValue("Nullable.null", fmt);
            }
            else
            {
                sink.formatValue(*_value, fmt);
            }
        }
    }

@system unittest
{
    import std.conv : to;

    const NullableRef!(ulong) x = new ulong(1);
    assert(x.to!string == "1");
}

/**
Binds the internal state to `value`.

Params:
    value = A pointer to a value of type `T` to bind this `NullableRef` to.
 */
    void bind(T* value) @safe pure nothrow
    {
        _value = value;
    }

    ///
    @safe unittest
    {
        NullableRef!int nr = new int(42);
        assert(nr == 42);

        int* n = new int(1);
        nr.bind(n);
        assert(nr == 1);
    }

/**
Returns `true` if and only if `this` is in the null state.

Returns:
    true if `this` is in the null state, otherwise false.
 */
    @property bool isNull() const @safe pure nothrow
    {
        return _value is null;
    }

    ///
    @safe unittest
    {
        NullableRef!int nr;
        assert(nr.isNull);

        int* n = new int(42);
        nr.bind(n);
        assert(!nr.isNull && nr == 42);
    }

/**
Forces `this` to the null state.
 */
    void nullify() @safe pure nothrow
    {
        _value = null;
    }

    ///
    @safe unittest
    {
        NullableRef!int nr = new int(42);
        assert(!nr.isNull);

        nr.nullify();
        assert(nr.isNull);
    }

/**
Assigns `value` to the internally-held state.

Params:
    value = A value of type `T` to assign to this `NullableRef`.
            If the internal state of this `NullableRef` has not
            been initialized, an error will be thrown in
            non-release mode.
 */
    void opAssign()(T value)
    if (isAssignable!T) //@@@9416@@@
    {
        enum message = "Called `opAssign' on null NullableRef!" ~ T.stringof ~ ".";
        assert(!isNull, message);
        *_value = value;
    }

    ///
    @system unittest
    {
        import std.exception : assertThrown, assertNotThrown;

        NullableRef!int nr;
        assert(nr.isNull);
        assertThrown!Throwable(nr = 42);

        nr.bind(new int(0));
        assert(!nr.isNull);
        assertNotThrown!Throwable(nr = 42);
        assert(nr == 42);
    }

/**
Gets the value. `this` must not be in the null state.
This function is also called for the implicit conversion to `T`.
 */
    @property ref inout(T) get() inout @safe pure nothrow
    {
        enum message = "Called `get' on null NullableRef!" ~ T.stringof ~ ".";
        assert(!isNull, message);
        return *_value;
    }

    ///
    @system unittest
    {
        import std.exception : assertThrown, assertNotThrown;

        NullableRef!int nr;
        //`get` is implicitly called. Will throw
        //an error in non-release mode
        assertThrown!Throwable(nr == 0);

        nr.bind(new int(0));
        assertNotThrown!Throwable(nr == 0);
    }

/**
Implicitly converts to `T`.
`this` must not be in the null state.
 */
    alias get this;
}

/// ditto
auto nullableRef(T)(T* t)
{
    return NullableRef!T(t);
}

///
@system unittest
{
    import std.exception : assertThrown;

    int x = 5, y = 7;
    auto a = nullableRef(&x);
    assert(!a.isNull);
    assert(a == 5);
    assert(x == 5);
    a = 42;
    assert(x == 42);
    assert(!a.isNull);
    assert(a == 42);
    a.nullify();
    assert(x == 42);
    assert(a.isNull);
    assertThrown!Throwable(a.get);
    assertThrown!Throwable(a = 71);
    a.bind(&y);
    assert(a == 7);
    y = 135;
    assert(a == 135);
}
@system unittest
{
    static int f(scope const NullableRef!int x) {
        return x.isNull ? 42 : x.get;
    }
    int x = 5;
    auto a = nullableRef(&x);
    assert(f(a) == 5);
    a.nullify();
    assert(f(a) == 42);
}
@safe unittest
{
    // Ensure NullableRef can be used in pure/nothrow/@safe environment.
    function() @safe pure nothrow
    {
        auto storage = new int;
        *storage = 19902;
        NullableRef!int n;
        assert(n.isNull);
        n.bind(storage);
        assert(!n.isNull);
        assert(n == 19902);
        n = 2294;
        assert(n == 2294);
        assert(*storage == 2294);
        n.nullify();
        assert(n.isNull);
    }();
}
@system unittest
{
    // Ensure NullableRef can be used when the value is not pure/nothrow/@safe
    static struct S
    {
        int x;
        this(this) @system {}
        bool opEquals(const S s) const @system { return s.x == x; }
    }

    auto storage = S(5);

    NullableRef!S s;
    assert(s.isNull);
    s.bind(&storage);
    assert(!s.isNull);
    assert(s.x == 5);
    s.nullify();
    assert(s.isNull);
}
@safe unittest
{
    //Check nullable is nicelly embedable in a struct
    static struct S1
    {
        NullableRef!int ni;
    }
    static struct S2 //inspired from 9404
    {
        NullableRef!int ni;
        this(S2 other)
        {
            ni = other.ni;
        }
        void opAssign(S2 other)
        {
            ni = other.ni;
        }
    }
    static foreach (S; AliasSeq!(S1, S2))
    {{
        S a;
        S b = a;
        S c;
        c = a;
    }}
}

// https://issues.dlang.org/show_bug.cgi?id=10915
@system unittest
{
    import std.conv : to;

    NullableRef!int nri;
    assert(nri.to!string() == "Nullable.null");

    struct Test
    {
        string s;
    }
    NullableRef!Test nt = new Test("test");
    assert(nt.to!string() == `Test("test")`);

    class TestToString
    {
        double d;

        this(double d)
        {
            this.d = d;
        }

        override string toString()
        {
            return d.to!string();
        }
    }
    TestToString tts = new TestToString(2.5);
    NullableRef!TestToString ntts = &tts;
    assert(ntts.to!string() == "2.5");
}


/**
`BlackHole!Base` is a subclass of `Base` which automatically implements
all abstract member functions in `Base` as do-nothing functions.  Each
auto-implemented function just returns the default value of the return type
without doing anything.

The name came from
$(HTTP search.cpan.org/~sburke/Class-_BlackHole-0.04/lib/Class/_BlackHole.pm, Class::_BlackHole)
Perl module by Sean M. Burke.

Params:
    Base = A non-final class for `BlackHole` to inherit from.

See_Also:
  $(LREF AutoImplement), $(LREF generateEmptyFunction)
 */
alias BlackHole(Base) = AutoImplement!(Base, generateEmptyFunction, isAbstractFunction);

///
@system unittest
{
    import std.math.traits : isNaN;

    static abstract class C
    {
        int m_value;
        this(int v) { m_value = v; }
        int value() @property { return m_value; }

        abstract real realValue() @property;
        abstract void doSomething();
    }

    auto c = new BlackHole!C(42);
    assert(c.value == 42);

    // Returns real.init which is NaN
    assert(c.realValue.isNaN);
    // Abstract functions are implemented as do-nothing
    c.doSomething();
}

@system unittest
{
    import std.math.traits : isNaN;

    // return default
    {
        interface I_1 { real test(); }
        auto o = new BlackHole!I_1;
        assert(o.test().isNaN()); // NaN
    }
    // doc example
    {
        static class C
        {
            int m_value;
            this(int v) { m_value = v; }
            int value() @property { return m_value; }

            abstract real realValue() @property;
            abstract void doSomething();
        }

        auto c = new BlackHole!C(42);
        assert(c.value == 42);

        assert(c.realValue.isNaN); // NaN
        c.doSomething();
    }

    // https://issues.dlang.org/show_bug.cgi?id=12058
    interface Foo
    {
        inout(Object) foo() inout;
    }
    BlackHole!Foo o;
}

nothrow pure @nogc @safe unittest
{
    static interface I
    {
        I foo() nothrow pure @nogc @safe return scope;
    }

    scope cb = new BlackHole!I();
    cb.foo();
}


/**
`WhiteHole!Base` is a subclass of `Base` which automatically implements
all abstract member functions as functions that always fail. These functions
simply throw an `Error` and never return. `Whitehole` is useful for
trapping the use of class member functions that haven't been implemented.

The name came from
$(HTTP search.cpan.org/~mschwern/Class-_WhiteHole-0.04/lib/Class/_WhiteHole.pm, Class::_WhiteHole)
Perl module by Michael G Schwern.

Params:
    Base = A non-final class for `WhiteHole` to inherit from.

See_Also:
  $(LREF AutoImplement), $(LREF generateAssertTrap)
 */
alias WhiteHole(Base) = AutoImplement!(Base, generateAssertTrap, isAbstractFunction);

///
@system unittest
{
    import std.exception : assertThrown;

    static class C
    {
        abstract void notYetImplemented();
    }

    auto c = new WhiteHole!C;
    assertThrown!NotImplementedError(c.notYetImplemented()); // throws an Error
}

// https://issues.dlang.org/show_bug.cgi?id=20232
nothrow pure @safe unittest
{
    static interface I
    {
        I foo() nothrow pure @safe return scope;
    }

    if (0) // Just checking attribute interference
    {
        scope cw = new WhiteHole!I();
        cw.foo();
    }
}

/// ditto
class NotImplementedError : Error
{
    ///
    this(string method) nothrow pure @safe
    {
        super(method ~ " is not implemented");
    }
}

///
@system unittest
{
    import std.exception : assertThrown;
    // nothrow
    {
        interface I_1
        {
            void foo();
            void bar() nothrow;
        }
        auto o = new WhiteHole!I_1;
        assertThrown!NotImplementedError(o.foo());
        assertThrown!NotImplementedError(o.bar());
    }
    // doc example
    {
        static class C
        {
            abstract void notYetImplemented();
        }

        auto c = new WhiteHole!C;
        try
        {
            c.notYetImplemented();
            assert(0);
        }
        catch (Error e) {}
    }
}


/**
`AutoImplement` automatically implements (by default) all abstract member
functions in the class or interface `Base` in specified way.

The second version of `AutoImplement` automatically implements
`Interface`, while deriving from `BaseClass`.

Params:
  how  = template which specifies _how functions will be implemented/overridden.

         Two arguments are passed to `how`: the type `Base` and an alias
         to an implemented function.  Then `how` must return an implemented
         function body as a string.

         The generated function body can use these keywords:
         $(UL
            $(LI `a0`, `a1`, &hellip;: arguments passed to the function;)
            $(LI `args`: a tuple of the arguments;)
            $(LI `self`: an alias to the function itself;)
            $(LI `parent`: an alias to the overridden function (if any).)
         )

        You may want to use templated property functions (instead of Implicit
        Template Properties) to generate complex functions:
--------------------
// Prints log messages for each call to overridden functions.
string generateLogger(C, alias fun)() @property
{
    import std.traits;
    enum qname = C.stringof ~ "." ~ __traits(identifier, fun);
    string stmt;

    stmt ~= q{ struct Importer { import std.stdio; } };
    stmt ~= `Importer.writeln("Log: ` ~ qname ~ `(", args, ")");`;
    static if (!__traits(isAbstractFunction, fun))
    {
        static if (is(ReturnType!fun == void))
            stmt ~= q{ parent(args); };
        else
            stmt ~= q{
                auto r = parent(args);
                Importer.writeln("--> ", r);
                return r;
            };
    }
    return stmt;
}
--------------------

  what = template which determines _what functions should be
         implemented/overridden.

         An argument is passed to `what`: an alias to a non-final member
         function in `Base`.  Then `what` must return a boolean value.
         Return `true` to indicate that the passed function should be
         implemented/overridden.

--------------------
// Sees if fun returns something.
enum bool hasValue(alias fun) = !is(ReturnType!(fun) == void);
--------------------


Note:

Generated code is inserted in the scope of `std.typecons` module.  Thus,
any useful functions outside `std.typecons` cannot be used in the generated
code.  To workaround this problem, you may `import` necessary things in a
local struct, as done in the `generateLogger()` template in the above
example.


BUGS:

$(UL
 $(LI Variadic arguments to constructors are not forwarded to super.)
 $(LI Deep interface inheritance causes compile error with messages like
      "Error: function std.typecons._AutoImplement!(Foo)._AutoImplement.bar
      does not override any function".  [$(BUGZILLA 2525)] )
 $(LI The `parent` keyword is actually a delegate to the super class'
      corresponding member function.  [$(BUGZILLA 2540)] )
 $(LI Using alias template parameter in `how` and/or `what` may cause
     strange compile error.  Use template tuple parameter instead to workaround
     this problem.  [$(BUGZILLA 4217)] )
)
 */
class AutoImplement(Base, alias how, alias what = isAbstractFunction) : Base
if (!is(how == class))
{
    private alias autoImplement_helper_ =
        AutoImplement_Helper!("autoImplement_helper_", "Base", Base, typeof(this), how, what);
    mixin(autoImplement_helper_.code);
}

/// ditto
class AutoImplement(
    Interface, BaseClass, alias how,
    alias what = isAbstractFunction) : BaseClass, Interface
if (is(Interface == interface) && is(BaseClass == class))
{
    private alias autoImplement_helper_ = AutoImplement_Helper!(
            "autoImplement_helper_", "Interface", Interface, typeof(this), how, what);
    mixin(autoImplement_helper_.code);
}

///
@system unittest
{
    interface PackageSupplier
    {
        int foo();
        int bar();
    }

    static abstract class AbstractFallbackPackageSupplier : PackageSupplier
    {
        protected PackageSupplier default_, fallback;

        this(PackageSupplier default_, PackageSupplier fallback)
        {
            this.default_ = default_;
            this.fallback = fallback;
        }

        abstract int foo();
        abstract int bar();
    }

    template fallback(T, alias func)
    {
        import std.format : format;
        // for all implemented methods:
        // - try default first
        // - only on a failure run & return fallback
        enum fallback = q{
            try
            {
                return default_.%1$s(args);
            }
            catch (Exception)
            {
                return fallback.%1$s(args);
            }
        }.format(__traits(identifier, func));
    }

    // combines two classes and use the second one as fallback
    alias FallbackPackageSupplier = AutoImplement!(AbstractFallbackPackageSupplier, fallback);

    class FailingPackageSupplier : PackageSupplier
    {
        int foo(){ throw new Exception("failure"); }
        int bar(){ return 2;}
    }

    class BackupPackageSupplier : PackageSupplier
    {
        int foo(){ return -1; }
        int bar(){ return -1;}
    }

    auto registry = new FallbackPackageSupplier(new FailingPackageSupplier(), new BackupPackageSupplier());

    assert(registry.foo() == -1);
    assert(registry.bar() == 2);
}

/*
 * Code-generating stuffs are encupsulated in this helper template so that
 * namespace pollution, which can cause name confliction with Base's public
 * members, should be minimized.
 */
private template AutoImplement_Helper(string myName, string baseName,
        Base, Self, alias generateMethodBody, alias cherrypickMethod)
{
private static:
    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://
    // Internal stuffs
    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://

    // Returns function overload sets in the class C, filtered with pred.
    template enumerateOverloads(C, alias pred)
    {
        template Impl(names...)
        {
            import std.meta : Filter;
            static if (names.length > 0)
            {
                alias methods = Filter!(pred, MemberFunctionsTuple!(C, names[0]));
                alias next = Impl!(names[1 .. $]);

                static if (methods.length > 0)
                    alias Impl = AliasSeq!(OverloadSet!(names[0], methods), next);
                else
                    alias Impl = next;
            }
            else
                alias Impl = AliasSeq!();
        }

        alias enumerateOverloads = Impl!(__traits(allMembers, C));
    }

    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://
    // Target functions
    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://

    // Add a non-final check to the cherrypickMethod.
    enum bool canonicalPicker(fun.../+[https://issues.dlang.org/show_bug.cgi?id=4217]+/) =
        !__traits(isFinalFunction, fun[0]) && cherrypickMethod!(fun);

    /*
     * A tuple of overload sets, each item of which consists of functions to be
     * implemented by the generated code.
     */
    alias targetOverloadSets = enumerateOverloads!(Base, canonicalPicker);

    /*
     * Super class of this AutoImplement instance
     */
    alias Super = BaseTypeTuple!(Self)[0];
    static assert(is(Super == class));
    static assert(is(Base == interface) || is(Super == Base));

    /*
     * A tuple of the super class' constructors.  Used for forwarding
     * constructor calls.
     */
    static if (__traits(hasMember, Super, "__ctor"))
        alias ctorOverloadSet = OverloadSet!("__ctor", __traits(getOverloads, Super, "__ctor"));
    else
        alias ctorOverloadSet = OverloadSet!("__ctor"); // empty


    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://
    // Type information
    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://

    /*
     * The generated code will be mixed into AutoImplement, which will be
     * instantiated in this module's scope.  Thus, any user-defined types are
     * out of scope and cannot be used directly (i.e. by their names).
     *
     * We will use FuncInfo instances for accessing return types and parameter
     * types of the implemented functions.  The instances will be populated to
     * the AutoImplement's scope in a certain way; see the populate() below.
     */

    // Returns the preferred identifier for the FuncInfo instance for the i-th
    // overloaded function with the name.
    template INTERNAL_FUNCINFO_ID(string name, size_t i)
    {
        import std.format : format;

        enum string INTERNAL_FUNCINFO_ID = format("F_%s_%s", name, i);
    }

    /*
     * Insert FuncInfo instances about all the target functions here.  This
     * enables the generated code to access type information via, for example,
     * "autoImplement_helper_.F_foo_1".
     */
    template populate(overloads...)
    {
        static if (overloads.length > 0)
        {
            mixin populate!(overloads[0].name, overloads[0].contents);
            mixin populate!(overloads[1 .. $]);
        }
    }
    template populate(string name, methods...)
    {
        static if (methods.length > 0)
        {
            mixin populate!(name, methods[0 .. $ - 1]);
            //
            alias target = methods[$ - 1];
            enum ith = methods.length - 1;
            mixin("alias " ~ INTERNAL_FUNCINFO_ID!(name, ith) ~ " = FuncInfo!target;");
        }
    }

    public mixin populate!(targetOverloadSets);
    public mixin populate!(  ctorOverloadSet );


    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://
    // Code-generating policies
    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://

    /* Common policy configurations for generating constructors and methods. */
    template CommonGeneratingPolicy()
    {
        // base class identifier which generated code should use
        enum string BASE_CLASS_ID = baseName;

        // FuncInfo instance identifier which generated code should use
        template FUNCINFO_ID(string name, size_t i)
        {
            enum string FUNCINFO_ID =
                myName ~ "." ~ INTERNAL_FUNCINFO_ID!(name, i);
        }
    }

    /* Policy configurations for generating constructors. */
    template ConstructorGeneratingPolicy()
    {
        mixin CommonGeneratingPolicy;

        /* Generates constructor body.  Just forward to the base class' one. */
        string generateFunctionBody(ctor.../+[https://issues.dlang.org/show_bug.cgi?id=4217]+/)() @property
        {
            enum varstyle = variadicFunctionStyle!(typeof(&ctor[0]));

            static if (varstyle & (Variadic.c | Variadic.d))
            {
                // the argptr-forwarding problem
                //pragma(msg, "Warning: AutoImplement!(", Base, ") ",
                //        "ignored variadic arguments to the constructor ",
                //        FunctionTypeOf!(typeof(&ctor[0])) );
            }
            return "super(args);";
        }
    }

    /* Policy configurations for genearting target methods. */
    template MethodGeneratingPolicy()
    {
        mixin CommonGeneratingPolicy;

        /* Geneartes method body. */
        string generateFunctionBody(func.../+[https://issues.dlang.org/show_bug.cgi?id=4217]+/)() @property
        {
            return generateMethodBody!(Base, func); // given
        }
    }


    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://
    // Generated code
    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://

    alias ConstructorGenerator = MemberFunctionGenerator!(ConstructorGeneratingPolicy!());
    alias MethodGenerator      = MemberFunctionGenerator!(MethodGeneratingPolicy!());

    public enum string code =
        ConstructorGenerator.generateCode!(  ctorOverloadSet ) ~ "\n" ~
             MethodGenerator.generateCode!(targetOverloadSets);

    debug (SHOW_GENERATED_CODE)
    {
        pragma(msg, "-------------------- < ", Base, " >");
        pragma(msg, code);
        pragma(msg, "--------------------");
    }
}

//debug = SHOW_GENERATED_CODE;
@system unittest
{
    import core.vararg;
    // no function to implement
    {
        interface I_1 {}
        auto o = new BlackHole!I_1;
    }
    // parameters
    {
        interface I_3 { void test(int, in int, out int, ref int, lazy int); }
        auto o = new BlackHole!I_3;
    }
    // use of user-defined type
    {
        struct S {}
        interface I_4 { S test(); }
        auto o = new BlackHole!I_4;
    }
    // overloads
    {
        interface I_5
        {
            void test(string);
            real test(real);
            int  test();
        }
        auto o = new BlackHole!I_5;
    }
    // constructor forwarding
    {
        static class C_6
        {
            this(int n) { assert(n == 42); }
            this(string s) { assert(s == "Deeee"); }
            this(...) {}
        }
        auto o1 = new BlackHole!C_6(42);
        auto o2 = new BlackHole!C_6("Deeee");
        auto o3 = new BlackHole!C_6(1, 2, 3, 4);
    }
    // attributes
    {
        interface I_7
        {
            ref int test_ref();
            int test_pure() pure;
            int test_nothrow() nothrow;
            int test_property() @property;
            int test_safe() @safe;
            int test_trusted() @trusted;
            int test_system() @system;
            int test_pure_nothrow() pure nothrow;
        }
        auto o = new BlackHole!I_7;
    }
    // storage classes
    {
        interface I_8
        {
            void test_const() const;
            void test_immutable() immutable;
            void test_shared() shared;
            void test_shared_const() shared const;
        }
        auto o = new BlackHole!I_8;
    }
    // use baseclass
    {
        static class C_9
        {
            private string foo_;

            this(string s) {
                foo_ = s;
            }

            protected string boilerplate() @property
            {
                return "Boilerplate stuff.";
            }

            public string foo() @property
            {
                return foo_;
            }
        }

        interface I_10
        {
            string testMethod(size_t);
        }

        static string generateTestMethod(C, alias fun)() @property
        {
            return "return this.boilerplate[0 .. a0];";
        }

        auto o = new AutoImplement!(I_10, C_9, generateTestMethod)("Testing");
        assert(o.testMethod(11) == "Boilerplate");
        assert(o.foo == "Testing");
    }
    /+ // deep inheritance
    {
    // https://issues.dlang.org/show_bug.cgi?id=2525
    // https://issues.dlang.org/show_bug.cgi?id=3525
    // NOTE: [r494] func.c(504-571) FuncDeclaration::semantic()
        interface I { void foo(); }
        interface J : I {}
        interface K : J {}
        static abstract class C_9 : K {}
        auto o = new BlackHole!C_9;
    }+/
    // test `parent` alias
    {
        interface I_11
        {
            void simple(int) @safe;
            int anotherSimple(string);
            int overloaded(int);
            /+ XXX [BUG 19715]
            void overloaded(string) @safe;
            +/
        }

        static class C_11
        {
            import std.traits : Parameters, ReturnType;
            import std.meta : Alias;

            protected ReturnType!fn _impl(alias fn)(Parameters!fn)
            if (is(Alias!(__traits(parent, fn)) == interface))
            {
                static if (!is(typeof(return) == void))
                    return typeof(return).init;
            }
        }

        template tpl(I, alias fn)
        if (is(I == interface) && __traits(isSame, __traits(parent, fn), I))
        {
            enum string tpl = q{
                enum bool haveReturn = !is(typeof(return) == void);

                static if (is(typeof(return) == void))
                    _impl!parent(args);
                else
                    return _impl!parent(args);
            };
        }

        auto o = new AutoImplement!(I_11, C_11, tpl);
    }
}

// https://issues.dlang.org/show_bug.cgi?id=17177
// AutoImplement fails on function overload sets with
// "cannot infer type from overloaded function symbol"
@system unittest
{
    static class Issue17177
    {
        private string n_;

        public {
            Issue17177 overloaded(string n)
            {
                this.n_ = n;

                return this;
            }

            string overloaded()
            {
                return this.n_;
            }
        }
    }

    static string how(C, alias fun)()
    {
        static if (!is(ReturnType!fun == void))
        {
            return q{
                return parent(args);
            };
        }
        else
        {
            return q{
                parent(args);
            };
        }
    }

    import std.meta : templateNot;
    alias Implementation = AutoImplement!(Issue17177, how, templateNot!isFinalFunction);
}

version (StdUnittest)
{
    // https://issues.dlang.org/show_bug.cgi?id=10647
    // Add prefix "issue10647_" as a workaround for
    // https://issues.dlang.org/show_bug.cgi?id=1238
    private string issue10647_generateDoNothing(C, alias fun)() @property
    {
        string stmt;

        static if (is(ReturnType!fun == void))
            stmt ~= "";
        else
        {
            string returnType = ReturnType!fun.stringof;
            stmt ~= "return "~returnType~".init;";
        }
        return stmt;
    }

    private template issue10647_isAlwaysTrue(alias fun)
    {
        enum issue10647_isAlwaysTrue = true;
    }

    // Do nothing template
    private template issue10647_DoNothing(Base)
    {
        alias issue10647_DoNothing = AutoImplement!(Base, issue10647_generateDoNothing, issue10647_isAlwaysTrue);
    }

    // A class to be overridden
    private class issue10647_Foo{
        void bar(int a) { }
    }
}

@system unittest
{
    auto foo = new issue10647_DoNothing!issue10647_Foo();
    foo.bar(13);
}

/*
Used by MemberFunctionGenerator.
 */
package template OverloadSet(string nam, T...)
{
    enum string name = nam;
    alias contents = T;
}

/*
Used by MemberFunctionGenerator.
 */
package template FuncInfo(alias func)
if (is(typeof(&func)))
{
    alias RT = ReturnType!(typeof(&func));
    alias PT = Parameters!(typeof(&func));
}
package template FuncInfo(Func)
{
    alias RT = ReturnType!Func;
    alias PT = Parameters!Func;
}

/*
General-purpose member function generator.
--------------------
template GeneratingPolicy()
{
    // [optional] the name of the class where functions are derived
    enum string BASE_CLASS_ID;

    // [optional] define this if you have only function types
    enum bool WITHOUT_SYMBOL;

    // [optional] Returns preferred identifier for i-th parameter.
    template PARAMETER_VARIABLE_ID(size_t i);

    // Returns the identifier of the FuncInfo instance for the i-th overload
    // of the specified name.  The identifier must be accessible in the scope
    // where generated code is mixed.
    template FUNCINFO_ID(string name, size_t i);

    // Returns implemented function body as a string.  When WITHOUT_SYMBOL is
    // defined, the latter is used.
    template generateFunctionBody(alias func);
    template generateFunctionBody(string name, FuncType);
}
--------------------
 */
package template MemberFunctionGenerator(alias Policy)
{
private static:
    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://
    // Internal stuffs
    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://
    import std.format;
    alias format = std.format.format;

    enum CONSTRUCTOR_NAME = "__ctor";

    // true if functions are derived from a base class
    enum WITH_BASE_CLASS = __traits(hasMember, Policy, "BASE_CLASS_ID");

    // true if functions are specified as types, not symbols
    enum WITHOUT_SYMBOL = __traits(hasMember, Policy, "WITHOUT_SYMBOL");

    // preferred identifier for i-th parameter variable
    static if (__traits(hasMember, Policy, "PARAMETER_VARIABLE_ID"))
    {
        alias PARAMETER_VARIABLE_ID = Policy.PARAMETER_VARIABLE_ID;
    }
    else
    {
        enum string PARAMETER_VARIABLE_ID(size_t i) = format("a%s", i);
            // default: a0, a1, ...
    }

    // Returns a tuple consisting of 0,1,2,...,n-1.  For static foreach.
    template CountUp(size_t n)
    {
        static if (n > 0)
            alias CountUp = AliasSeq!(CountUp!(n - 1), n - 1);
        else
            alias CountUp = AliasSeq!();
    }


    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://
    // Code generator
    //:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::://

    /*
     * Runs through all the target overload sets and generates D code which
     * implements all the functions in the overload sets.
     */
    public string generateCode(overloads...)() @property
    {
        string code = "";

        // run through all the overload sets
        foreach (i_; CountUp!(0 + overloads.length)) // workaround
        {
            enum i = 0 + i_; // workaround
            alias oset = overloads[i];

            code ~= generateCodeForOverloadSet!(oset);

            static if (WITH_BASE_CLASS && oset.name != CONSTRUCTOR_NAME)
            {
                // The generated function declarations may hide existing ones
                // in the base class (cf. HiddenFuncError), so we put an alias
                // declaration here to reveal possible hidden functions.
                code ~= format("alias %s = %s.%s;\n",
                            oset.name,
                            // super: https://issues.dlang.org/show_bug.cgi?id=2540
                            Policy.BASE_CLASS_ID,
                            oset.name);
            }
        }
        return code;
    }

    // handle each overload set
    string generateCodeForOverloadSet(alias oset)() @property
    {
        string code = "";

        foreach (i_; CountUp!(0 + oset.contents.length)) // workaround
        {
            enum i = 0 + i_; // workaround
            code ~= generateFunction!(
                    Policy.FUNCINFO_ID!(oset.name, i), oset.name,
                    oset.contents[i]) ~ "\n";
        }
        return code;
    }

    /*
     * Returns D code which implements the function func.  This function
     * actually generates only the declarator part; the function body part is
     * generated by the functionGenerator() policy.
     */
    public string generateFunction(
            string myFuncInfo, string name, func... )() @property
    {
        import std.format : format;

        enum isCtor = (name == CONSTRUCTOR_NAME);

        string code; // the result

        auto paramsRes = generateParameters!(myFuncInfo, func)();
        code ~= paramsRes.imports;

        /*** Function Declarator ***/
        {
            alias Func = FunctionTypeOf!(func);
            alias FA = FunctionAttribute;
            enum atts     = functionAttributes!(func);
            enum realName = isCtor ? "this" : name;

            // FIXME?? Make it so that these aren't CTFE funcs any more, since
            // Format is deprecated, and format works at compile time?
            /* Made them CTFE funcs just for the sake of Format!(...) */

            // return type with optional "ref"
            static string make_returnType()
            {
                string rtype = "";

                if (!isCtor)
                {
                    if (atts & FA.ref_) rtype ~= "ref ";
                    rtype ~= myFuncInfo ~ ".RT";
                }
                return rtype;
            }
            enum returnType = make_returnType();

            // function attributes attached after declaration
            static string make_postAtts()
            {
                string poatts = "";
                if (atts & FA.pure_   ) poatts ~= " pure";
                if (atts & FA.nothrow_) poatts ~= " nothrow";
                if (atts & FA.property) poatts ~= " @property";
                if (atts & FA.safe    ) poatts ~= " @safe";
                if (atts & FA.trusted ) poatts ~= " @trusted";
                if (atts & FA.scope_ )  poatts ~= " scope";
                if (atts & FA.return_ ) poatts ~= " return";
                return poatts;
            }
            enum postAtts = make_postAtts();

            // function storage class
            static string make_storageClass()
            {
                string postc = "";
                if (is(Func ==    shared)) postc ~= " shared";
                if (is(Func ==     const)) postc ~= " const";
                if (is(Func ==     inout)) postc ~= " inout";
                if (is(Func == immutable)) postc ~= " immutable";
                return postc;
            }
            enum storageClass = make_storageClass();

            //
            if (__traits(isVirtualMethod, func))
                code ~= "override ";
            code ~= format("extern(%s) %s %s(%s) %s %s\n",
                    functionLinkage!(func),
                    returnType,
                    realName,
                    paramsRes.params,
                    postAtts, storageClass );
        }

        /*** Function Body ***/
        code ~= "{\n";
        {
            enum nparams = Parameters!(func).length;

            /* Declare keywords: args, self and parent. */
            string preamble;

            preamble ~= "alias args = AliasSeq!(" ~ enumerateParameters!(nparams) ~ ");\n";
            if (!isCtor)
            {
                preamble ~= "alias self = " ~ name ~ ";\n";
                static if (WITH_BASE_CLASS)
                    preamble ~= `alias parent = __traits(getMember, ` ~ Policy.BASE_CLASS_ID ~ `, "` ~ name ~ `");`;
            }

            // Function body
            static if (WITHOUT_SYMBOL)
                enum fbody = Policy.generateFunctionBody!(name, func);
            else
                enum fbody = Policy.generateFunctionBody!(func);

            code ~= preamble;
            code ~= fbody;
        }
        code ~= "}";

        return code;
    }

    /*
     * Returns D code which declares function parameters,
     * and optionally any imports (e.g. core.vararg)
     * "ref int a0, real a1, ..."
     */
    static struct GenParams { string imports, params; }
    GenParams generateParameters(string myFuncInfo, func...)()
    {
        alias STC = ParameterStorageClass;
        alias stcs = ParameterStorageClassTuple!(func);
        enum nparams = stcs.length;

        string imports = ""; // any imports required
        string params = ""; // parameters

        foreach (i, stc; stcs)
        {
            if (i > 0) params ~= ", ";

            // Parameter storage classes.
            if (stc & STC.scope_) params ~= "scope ";
            if (stc & STC.in_)    params ~= "in ";
            if (stc & STC.out_  ) params ~= "out ";
            if (stc & STC.ref_  ) params ~= "ref ";
            if (stc & STC.lazy_ ) params ~= "lazy ";

            // Take parameter type from the FuncInfo.
            params ~= format("%s.PT[%s]", myFuncInfo, i);

            // Declare a parameter variable.
            params ~= " " ~ PARAMETER_VARIABLE_ID!(i);
        }

        // Add some ellipsis part if needed.
        auto style = variadicFunctionStyle!(func);
        final switch (style)
        {
            case Variadic.no:
                break;

            case Variadic.c, Variadic.d:
                imports ~= "import core.vararg;\n";
                // (...) or (a, b, ...)
                params ~= (nparams == 0) ? "..." : ", ...";
                break;

            case Variadic.typesafe:
                params ~= " ...";
                break;
        }

        return typeof(return)(imports, params);
    }

    // Returns D code which enumerates n parameter variables using comma as the
    // separator.  "a0, a1, a2, a3"
    string enumerateParameters(size_t n)() @property
    {
        string params = "";

        foreach (i_; CountUp!(n))
        {
            enum i = 0 + i_; // workaround
            if (i > 0) params ~= ", ";
            params ~= PARAMETER_VARIABLE_ID!(i);
        }
        return params;
    }
}


/**
Predefined how-policies for `AutoImplement`.  These templates are also used by
`BlackHole` and `WhiteHole`, respectively.
 */
template generateEmptyFunction(C, func.../+[https://issues.dlang.org/show_bug.cgi?id=4217]+/)
{
    static if (is(ReturnType!(func) == void))
        enum string generateEmptyFunction = q{
        };
    else static if (functionAttributes!(func) & FunctionAttribute.ref_)
        enum string generateEmptyFunction = q{
            static typeof(return) dummy;
            return dummy;
        };
    else
        enum string generateEmptyFunction = q{
            return typeof(return).init;
        };
}

///
@system unittest
{
    alias BlackHole(Base) = AutoImplement!(Base, generateEmptyFunction);

    interface I
    {
        int foo();
        string bar();
    }

    auto i = new BlackHole!I();
    // generateEmptyFunction returns the default value of the return type without doing anything
    assert(i.foo == 0);
    assert(i.bar is null);
}

/// ditto
template generateAssertTrap(C, func...)
{
    enum string generateAssertTrap =
        `throw new NotImplementedError("` ~ C.stringof ~ "."
                ~ __traits(identifier, func) ~ `");`;
}

///
@system unittest
{
    import std.exception : assertThrown;

    alias WhiteHole(Base) = AutoImplement!(Base, generateAssertTrap);

    interface I
    {
        int foo();
        string bar();
    }

    auto i = new WhiteHole!I();
    // generateAssertTrap throws an exception for every unimplemented function of the interface
    assertThrown!NotImplementedError(i.foo);
    assertThrown!NotImplementedError(i.bar);
}

private
{
    pragma(mangle, "_d_toObject")
    extern(C) pure nothrow Object typecons_d_toObject(void* p);
}

/*
 * Avoids opCast operator overloading.
 */
private template dynamicCast(T)
if (is(T == class) || is(T == interface))
{
    @trusted
    T dynamicCast(S)(inout S source)
    if (is(S == class) || is(S == interface))
    {
        static if (is(Unqual!S : Unqual!T))
        {
            import std.traits : QualifierOf;
            alias Qual = QualifierOf!S; // SharedOf or MutableOf
            alias TmpT = Qual!(Unqual!T);
            inout(TmpT) tmp = source;   // bypass opCast by implicit conversion
            return *cast(T*)(&tmp);     // + variable pointer cast + dereference
        }
        else
        {
            return cast(T) typecons_d_toObject(*cast(void**)(&source));
        }
    }
}

@system unittest
{
    class C { @disable void opCast(T)(); }
    auto c = new C;
    static assert(!__traits(compiles, cast(Object) c));
    auto o = dynamicCast!Object(c);
    assert(c is o);

    interface I { @disable void opCast(T)(); Object instance(); }
    interface J { @disable void opCast(T)(); Object instance(); }
    class D : I, J { Object instance() { return this; } }
    I i = new D();
    static assert(!__traits(compiles, cast(J) i));
    J j = dynamicCast!J(i);
    assert(i.instance() is j.instance());
}

/**
Supports structural based typesafe conversion.

If `Source` has structural conformance with the `interface` `Targets`,
wrap creates an internal wrapper class which inherits `Targets` and
wraps the `src` object, then returns it.

`unwrap` can be used to extract objects which have been wrapped by `wrap`.
*/
template wrap(Targets...)
if (Targets.length >= 1 && allSatisfy!(isMutable, Targets))
{
    import std.meta : staticMap;

    // strict upcast
    auto wrap(Source)(inout Source src) @trusted pure nothrow
    if (Targets.length == 1 && is(Source : Targets[0]))
    {
        alias T = Select!(is(Source == shared), shared Targets[0], Targets[0]);
        return dynamicCast!(inout T)(src);
    }
    // structural upcast
    template wrap(Source)
    if (!allSatisfy!(Bind!(isImplicitlyConvertible, Source), Targets))
    {
        auto wrap(inout Source src)
        {
            static assert(hasRequireMethods!(),
                          "Source "~Source.stringof~
                          " does not have structural conformance to "~
                          Targets.stringof);

            alias T = Select!(is(Source == shared), shared Impl, Impl);
            return new inout T(src);
        }

        template FuncInfo(string s, F)
        {
            enum name = s;
            alias type = F;
        }

        // https://issues.dlang.org/show_bug.cgi?id=12064: Remove NVI members
        template OnlyVirtual(members...)
        {
            enum notFinal(alias T) = !__traits(isFinalFunction, T);
            import std.meta : Filter;
            alias OnlyVirtual = Filter!(notFinal, members);
        }

        // Concat all Targets function members into one tuple
        template Concat(size_t i = 0)
        {
            static if (i >= Targets.length)
                alias Concat = AliasSeq!();
            else
            {
                alias Concat = AliasSeq!(OnlyVirtual!(GetOverloadedMethods!(Targets[i]), Concat!(i + 1)));
            }
        }

        // Remove duplicated functions based on the identifier name and function type covariance
        template Uniq(members...)
        {
            static if (members.length == 0)
                alias Uniq = AliasSeq!();
            else
            {
                alias func = members[0];
                enum  name = __traits(identifier, func);
                alias type = FunctionTypeOf!func;
                template check(size_t i, mem...)
                {
                    static if (i >= mem.length)
                        enum ptrdiff_t check = -1;
                    else
                    {
                        enum ptrdiff_t check =
                            __traits(identifier, func) == __traits(identifier, mem[i]) &&
                            !is(DerivedFunctionType!(type, FunctionTypeOf!(mem[i])) == void)
                          ? i : check!(i + 1, mem);
                    }
                }
                enum ptrdiff_t x = 1 + check!(0, members[1 .. $]);
                static if (x >= 1)
                {
                    alias typex = DerivedFunctionType!(type, FunctionTypeOf!(members[x]));
                    alias remain = Uniq!(members[1 .. x], members[x + 1 .. $]);

                    static if (remain.length >= 1 && remain[0].name == name &&
                               !is(DerivedFunctionType!(typex, remain[0].type) == void))
                    {
                        alias F = DerivedFunctionType!(typex, remain[0].type);
                        alias Uniq = AliasSeq!(FuncInfo!(name, F), remain[1 .. $]);
                    }
                    else
                        alias Uniq = AliasSeq!(FuncInfo!(name, typex), remain);
                }
                else
                {
                    alias Uniq = AliasSeq!(FuncInfo!(name, type), Uniq!(members[1 .. $]));
                }
            }
        }
        alias TargetMembers = Uniq!(Concat!());             // list of FuncInfo
        alias SourceMembers = GetOverloadedMethods!Source;  // list of function symbols

        // Check whether all of SourceMembers satisfy covariance target in TargetMembers
        template hasRequireMethods(size_t i = 0)
        {
            static if (i >= TargetMembers.length)
                enum hasRequireMethods = true;
            else
            {
                enum hasRequireMethods =
                    findCovariantFunction!(TargetMembers[i], Source, SourceMembers) != -1 &&
                    hasRequireMethods!(i + 1);
            }
        }

        // Internal wrapper class
        final class Impl : Structural, Targets
        {
        private:
            Source _wrap_source;

            this(       inout Source s)        inout @safe pure nothrow { _wrap_source = s; }
            this(shared inout Source s) shared inout @safe pure nothrow { _wrap_source = s; }

            // BUG: making private should work with NVI.
            protected final inout(Object) _wrap_getSource() inout @trusted
            {
                return dynamicCast!(inout Object)(_wrap_source);
            }

            import std.conv : to;
            import core.lifetime : forward;
            template generateFun(size_t i)
            {
                enum name = TargetMembers[i].name;
                enum fa = functionAttributes!(TargetMembers[i].type);
                static @property stc()
                {
                    string r;
                    if (fa & FunctionAttribute.property)    r ~= "@property ";
                    if (fa & FunctionAttribute.ref_)        r ~= "ref ";
                    if (fa & FunctionAttribute.pure_)       r ~= "pure ";
                    if (fa & FunctionAttribute.nothrow_)    r ~= "nothrow ";
                    if (fa & FunctionAttribute.trusted)     r ~= "@trusted ";
                    if (fa & FunctionAttribute.safe)        r ~= "@safe ";
                    return r;
                }
                static @property mod()
                {
                    alias type = AliasSeq!(TargetMembers[i].type)[0];
                    string r;
                    static if (is(type == immutable))       r ~= " immutable";
                    else
                    {
                        static if (is(type == shared))      r ~= " shared";
                        static if (is(type == const))       r ~= " const";
                        else static if (is(type == inout))  r ~= " inout";
                        //else  --> mutable
                    }
                    return r;
                }
                enum n = to!string(i);
                static if (fa & FunctionAttribute.property)
                {
                    static if (Parameters!(TargetMembers[i].type).length == 0)
                        enum fbody = "_wrap_source."~name;
                    else
                        enum fbody = "_wrap_source."~name~" = forward!args";
                }
                else
                {
                        enum fbody = "_wrap_source."~name~"(forward!args)";
                }
                enum generateFun =
                    "override "~stc~"ReturnType!(TargetMembers["~n~"].type) "
                    ~ name~"(Parameters!(TargetMembers["~n~"].type) args) "~mod~
                    "{ return "~fbody~"; }";
            }

        public:
            static foreach (i; 0 .. TargetMembers.length)
                mixin(generateFun!i);
        }
    }
}
/// ditto
template wrap(Targets...)
if (Targets.length >= 1 && !allSatisfy!(isMutable, Targets))
{
    import std.meta : staticMap;

    alias wrap = .wrap!(staticMap!(Unqual, Targets));
}

/// ditto
template unwrap(Target)
if (isMutable!Target)
{
    // strict downcast
    auto unwrap(Source)(inout Source src) @trusted pure nothrow
    if (is(Target : Source))
    {
        alias T = Select!(is(Source == shared), shared Target, Target);
        return dynamicCast!(inout T)(src);
    }
    // structural downcast
    auto unwrap(Source)(inout Source src) @trusted pure nothrow
    if (!is(Target : Source))
    {
        alias T = Select!(is(Source == shared), shared Target, Target);
        Object o = dynamicCast!(Object)(src);   // remove qualifier
        do
        {
            if (auto a = dynamicCast!(Structural)(o))
            {
                if (auto d = dynamicCast!(inout T)(o = a._wrap_getSource()))
                    return d;
            }
            else if (auto d = dynamicCast!(inout T)(o))
                return d;
            else
                break;
        } while (o);
        return null;
    }
}

/// ditto
template unwrap(Target)
if (!isMutable!Target)
{
    alias unwrap = .unwrap!(Unqual!Target);
}

///
@system unittest
{
    interface Quack
    {
        int quack();
        @property int height();
    }
    interface Flyer
    {
        @property int height();
    }
    class Duck : Quack
    {
        int quack() { return 1; }
        @property int height() { return 10; }
    }
    class Human
    {
        int quack() { return 2; }
        @property int height() { return 20; }
    }

    Duck d1 = new Duck();
    Human h1 = new Human();

    interface Refleshable
    {
        int reflesh();
    }

    // does not have structural conformance
    static assert(!__traits(compiles, d1.wrap!Refleshable));
    static assert(!__traits(compiles, h1.wrap!Refleshable));

    // strict upcast
    Quack qd = d1.wrap!Quack;
    assert(qd is d1);
    assert(qd.quack() == 1);    // calls Duck.quack
    // strict downcast
    Duck d2 = qd.unwrap!Duck;
    assert(d2 is d1);

    // structural upcast
    Quack qh = h1.wrap!Quack;
    assert(qh.quack() == 2);    // calls Human.quack
    // structural downcast
    Human h2 = qh.unwrap!Human;
    assert(h2 is h1);

    // structural upcast (two steps)
    Quack qx = h1.wrap!Quack;   // Human -> Quack
    Flyer fx = qx.wrap!Flyer;   // Quack -> Flyer
    assert(fx.height == 20);    // calls Human.height
    // structural downcast (two steps)
    Quack qy = fx.unwrap!Quack; // Flyer -> Quack
    Human hy = qy.unwrap!Human; // Quack -> Human
    assert(hy is h1);
    // structural downcast (one step)
    Human hz = fx.unwrap!Human; // Flyer -> Human
    assert(hz is h1);
}

///
@system unittest
{
    import std.traits : FunctionAttribute, functionAttributes;
    interface A { int run(); }
    interface B { int stop(); @property int status(); }
    class X
    {
        int run() { return 1; }
        int stop() { return 2; }
        @property int status() { return 3; }
    }

    auto x = new X();
    auto ab = x.wrap!(A, B);
    A a = ab;
    B b = ab;
    assert(a.run() == 1);
    assert(b.stop() == 2);
    assert(b.status == 3);
    static assert(functionAttributes!(typeof(ab).status) & FunctionAttribute.property);
}

// Internal class to support dynamic cross-casting
private interface Structural
{
    inout(Object) _wrap_getSource() inout @safe pure nothrow;
}

@system unittest
{
    class A
    {
        int draw()              { return 1; }
        int draw(int v)         { return v; }

        int draw() const        { return 2; }
        int draw() shared       { return 3; }
        int draw() shared const { return 4; }
        int draw() immutable    { return 5; }
    }
    interface Drawable
    {
        int draw();
        int draw() const;
        int draw() shared;
        int draw() shared const;
        int draw() immutable;
    }
    interface Drawable2
    {
        int draw(int v);
    }

    auto ma = new A();
    auto sa = new shared A();
    auto ia = new immutable A();
    {
                     Drawable  md = ma.wrap!Drawable;
               const Drawable  cd = ma.wrap!Drawable;
              shared Drawable  sd = sa.wrap!Drawable;
        shared const Drawable scd = sa.wrap!Drawable;
           immutable Drawable  id = ia.wrap!Drawable;
        assert( md.draw() == 1);
        assert( cd.draw() == 2);
        assert( sd.draw() == 3);
        assert(scd.draw() == 4);
        assert( id.draw() == 5);
    }
    {
        Drawable2 d = ma.wrap!Drawable2;
        static assert(!__traits(compiles, d.draw()));
        assert(d.draw(10) == 10);
    }
}

// https://issues.dlang.org/show_bug.cgi?id=10377
@system unittest
{
    import std.range, std.algorithm;

    interface MyInputRange(T)
    {
        @property T front();
        void popFront();
        @property bool empty();
    }

    //auto o = iota(0,10,1).inputRangeObject();
    //pragma(msg, __traits(allMembers, typeof(o)));
    auto r = iota(0,10,1).inputRangeObject().wrap!(MyInputRange!int)();
    assert(equal(r, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]));
}

// https://issues.dlang.org/show_bug.cgi?id=10536
@system unittest
{
    interface Interface
    {
        int foo();
    }
    class Pluggable
    {
        int foo() { return 1; }
        @disable void opCast(T, this X)();  // !
    }

    Interface i = new Pluggable().wrap!Interface;
    assert(i.foo() == 1);
}
@system unittest
{
    // Enhancement 10538
    interface Interface
    {
        int foo();
        int bar(int);
    }
    class Pluggable
    {
        int opDispatch(string name, A...)(A args) { return 100; }
    }

    Interface i = wrap!Interface(new Pluggable());
    assert(i.foo() == 100);
    assert(i.bar(10) == 100);
}

// https://issues.dlang.org/show_bug.cgi?id=12064
@system unittest
{
    interface I
    {
        int foo();
        final int nvi1(){return foo();}
    }

    interface J
    {
        int bar();
        final int nvi2(){return bar();}
    }

    class Baz
    {
        int foo() { return 42;}
        int bar() { return 12064;}
    }

    auto baz = new Baz();
    auto foobar = baz.wrap!(I, J)();
    assert(foobar.nvi1 == 42);
    assert(foobar.nvi2 == 12064);
}

// Make a tuple of non-static function symbols
package template GetOverloadedMethods(T)
{
    import std.meta : Filter;

    alias allMembers = __traits(allMembers, T);
    template follows(size_t i = 0)
    {
        static if (i >= allMembers.length)
        {
            alias follows = AliasSeq!();
        }
        else static if (!__traits(compiles, mixin("T."~allMembers[i])))
        {
            alias follows = follows!(i + 1);
        }
        else
        {
            enum name = allMembers[i];

            template isMethod(alias f)
            {
                static if (is(typeof(&f) F == F*) && is(F == function))
                    enum isMethod = !__traits(isStaticFunction, f);
                else
                    enum isMethod = false;
            }
            alias follows = AliasSeq!(
                Filter!(isMethod, __traits(getOverloads, T, name)),
                follows!(i + 1));
        }
    }
    alias GetOverloadedMethods = follows!();
}
// find a function from Fs that has same identifier and covariant type with f
private template findCovariantFunction(alias finfo, Source, Fs...)
{
    template check(size_t i = 0)
    {
        static if (i >= Fs.length)
            enum ptrdiff_t check = -1;
        else
        {
            enum ptrdiff_t check =
                (finfo.name == __traits(identifier, Fs[i])) &&
                isCovariantWith!(FunctionTypeOf!(Fs[i]), finfo.type)
              ? i : check!(i + 1);
        }
    }
    enum x = check!();
    static if (x == -1 && is(typeof(Source.opDispatch)))
    {
        alias Params = Parameters!(finfo.type);
        enum ptrdiff_t findCovariantFunction =
            is(typeof((             Source).init.opDispatch!(finfo.name)(Params.init))) ||
            is(typeof((       const Source).init.opDispatch!(finfo.name)(Params.init))) ||
            is(typeof((   immutable Source).init.opDispatch!(finfo.name)(Params.init))) ||
            is(typeof((      shared Source).init.opDispatch!(finfo.name)(Params.init))) ||
            is(typeof((shared const Source).init.opDispatch!(finfo.name)(Params.init)))
          ? ptrdiff_t.max : -1;
    }
    else
        enum ptrdiff_t findCovariantFunction = x;
}

private enum TypeModifier
{
    mutable     = 0,    // type is mutable
    const_      = 1,    // type is const
    immutable_  = 2,    // type is immutable
    shared_     = 4,    // type is shared
    inout_      = 8,    // type is wild
}
private template TypeMod(T)
{
    static if (is(T == immutable))
    {
        enum mod1 = TypeModifier.immutable_;
        enum mod2 = 0;
    }
    else
    {
        enum mod1 = is(T == shared) ? TypeModifier.shared_ : 0;
        static if (is(T == const))
            enum mod2 = TypeModifier.const_;
        else static if (is(T == inout))
            enum mod2 = TypeModifier.inout_;
        else
            enum mod2 = TypeModifier.mutable;
    }
    enum TypeMod = cast(TypeModifier)(mod1 | mod2);
}

@system unittest
{
    template UnittestFuncInfo(alias f)
    {
        enum name = __traits(identifier, f);
        alias type = FunctionTypeOf!f;
    }

    class A
    {
        int draw() { return 1; }
        @property int value() { return 2; }
        final int run() { return 3; }
    }
    alias methods = GetOverloadedMethods!A;

    alias int F1();
    alias @property int F2();
    alias string F3();
    alias nothrow @trusted uint F4();
    alias int F5(Object);
    alias bool F6(Object);
    static assert(methods.length == 3 + 4);
    static assert(__traits(identifier, methods[0]) == "draw"     && is(typeof(&methods[0]) == F1*));
    static assert(__traits(identifier, methods[1]) == "value"    && is(typeof(&methods[1]) == F2*));
    static assert(__traits(identifier, methods[2]) == "run"      && is(typeof(&methods[2]) == F1*));

    int draw();
    @property int value();
    void opEquals();
    int nomatch();
    static assert(findCovariantFunction!(UnittestFuncInfo!draw,     A, methods) == 0);
    static assert(findCovariantFunction!(UnittestFuncInfo!value,    A, methods) == 1);
    static assert(findCovariantFunction!(UnittestFuncInfo!opEquals, A, methods) == -1);
    static assert(findCovariantFunction!(UnittestFuncInfo!nomatch,  A, methods) == -1);

    // considering opDispatch
    class B
    {
        void opDispatch(string name, A...)(A) {}
    }
    alias methodsB = GetOverloadedMethods!B;
    static assert(findCovariantFunction!(UnittestFuncInfo!draw,     B, methodsB) == ptrdiff_t.max);
    static assert(findCovariantFunction!(UnittestFuncInfo!value,    B, methodsB) == ptrdiff_t.max);
    static assert(findCovariantFunction!(UnittestFuncInfo!opEquals, B, methodsB) == ptrdiff_t.max);
    static assert(findCovariantFunction!(UnittestFuncInfo!nomatch,  B, methodsB) == ptrdiff_t.max);
}

package template DerivedFunctionType(T...)
{
    static if (!T.length)
    {
        alias DerivedFunctionType = void;
    }
    else static if (T.length == 1)
    {
        static if (is(T[0] == function))
        {
            alias DerivedFunctionType = T[0];
        }
        else
        {
            alias DerivedFunctionType = void;
        }
    }
    else static if (is(T[0] P0 == function) && is(T[1] P1 == function))
    {
        alias FA = FunctionAttribute;

        alias F0 = T[0], R0 = ReturnType!F0, PSTC0 = ParameterStorageClassTuple!F0;
        alias F1 = T[1], R1 = ReturnType!F1, PSTC1 = ParameterStorageClassTuple!F1;
        enum FA0 = functionAttributes!F0;
        enum FA1 = functionAttributes!F1;

        template CheckParams(size_t i = 0)
        {
            static if (i >= P0.length)
                enum CheckParams = true;
            else
            {
                enum CheckParams = (is(P0[i] == P1[i]) && PSTC0[i] == PSTC1[i]) &&
                                   CheckParams!(i + 1);
            }
        }
        static if (R0.sizeof == R1.sizeof && !is(CommonType!(R0, R1) == void) &&
                   P0.length == P1.length && CheckParams!() && TypeMod!F0 == TypeMod!F1 &&
                   variadicFunctionStyle!F0 == variadicFunctionStyle!F1 &&
                   functionLinkage!F0 == functionLinkage!F1 &&
                   ((FA0 ^ FA1) & (FA.ref_ | FA.property)) == 0)
        {
            alias R = Select!(is(R0 : R1), R0, R1);
            alias FX = FunctionTypeOf!(R function(P0));
            // @system is default
            alias FY = SetFunctionAttributes!(FX, functionLinkage!F0, (FA0 | FA1) & ~FA.system);
            alias DerivedFunctionType = DerivedFunctionType!(FY, T[2 .. $]);
        }
        else
            alias DerivedFunctionType = void;
    }
    else
        alias DerivedFunctionType = void;
}
@safe unittest
{
    // attribute covariance
    alias int F1();
    static assert(is(DerivedFunctionType!(F1, F1) == F1));
    alias int F2() pure nothrow;
    static assert(is(DerivedFunctionType!(F1, F2) == F2));
    alias int F3() @safe;
    alias int F23() @safe pure nothrow;
    static assert(is(DerivedFunctionType!(F2, F3) == F23));

    // return type covariance
    alias long F4();
    static assert(is(DerivedFunctionType!(F1, F4) == void));
    class C {}
    class D : C {}
    alias C F5();
    alias D F6();
    static assert(is(DerivedFunctionType!(F5, F6) == F6));
    alias typeof(null) F7();
    alias int[] F8();
    alias int* F9();
    static assert(is(DerivedFunctionType!(F5, F7) == F7));
    static assert(is(DerivedFunctionType!(F7, F8) == void));
    static assert(is(DerivedFunctionType!(F7, F9) == F7));

    // variadic type equality
    alias int F10(int);
    alias int F11(int...);
    alias int F12(int, ...);
    static assert(is(DerivedFunctionType!(F10, F11) == void));
    static assert(is(DerivedFunctionType!(F10, F12) == void));
    static assert(is(DerivedFunctionType!(F11, F12) == void));

    // linkage equality
    alias extern(C) int F13(int);
    alias extern(D) int F14(int);
    alias extern(Windows) int F15(int);
    static assert(is(DerivedFunctionType!(F13, F14) == void));
    static assert(is(DerivedFunctionType!(F13, F15) == void));
    static assert(is(DerivedFunctionType!(F14, F15) == void));

    // ref & @property equality
    alias int F16(int);
    alias ref int F17(int);
    alias @property int F18(int);
    static assert(is(DerivedFunctionType!(F16, F17) == void));
    static assert(is(DerivedFunctionType!(F16, F18) == void));
    static assert(is(DerivedFunctionType!(F17, F18) == void));
}

package template Bind(alias Template, args1...)
{
    alias Bind(args2...) = Template!(args1, args2);
}


/**
Options regarding auto-initialization of a `SafeRefCounted` object (see
the definition of `SafeRefCounted` below).
 */
enum RefCountedAutoInitialize
{
    /// Do not auto-initialize the object
    no,
    /// Auto-initialize the object
    yes,
}

///
@system unittest
{
    import core.exception : AssertError;
    import std.exception : assertThrown;

    struct Foo
    {
        int a = 42;
    }

    SafeRefCounted!(Foo, RefCountedAutoInitialize.yes) rcAuto;
    SafeRefCounted!(Foo, RefCountedAutoInitialize.no) rcNoAuto;

    assert(rcAuto.refCountedPayload.a == 42);

    assertThrown!AssertError(rcNoAuto.refCountedPayload);
    rcNoAuto.refCountedStore.ensureInitialized;
    assert(rcNoAuto.refCountedPayload.a == 42);
}

// Same the above but for old RefCounted and not documented
@system unittest
{
    import core.exception : AssertError;
    import std.exception : assertThrown;

    struct Foo
    {
        int a = 42;
    }

    RefCounted!(Foo, RefCountedAutoInitialize.yes) rcAuto;
    RefCounted!(Foo, RefCountedAutoInitialize.no) rcNoAuto;

    assert(rcAuto.refCountedPayload.a == 42);

    assertThrown!AssertError(rcNoAuto.refCountedPayload);
    rcNoAuto.refCountedStore.ensureInitialized;
    assert(rcNoAuto.refCountedPayload.a == 42);
}

/**
Defines a reference-counted object containing a `T` value as
payload.

An instance of `SafeRefCounted` is a reference to a structure,
which is referred to as the $(I store), or $(I storage implementation
struct) in this documentation.  The store contains a reference count
and the `T` payload.  `SafeRefCounted` uses `malloc` to allocate
the store.  As instances of `SafeRefCounted` are copied or go out of
scope, they will automatically increment or decrement the reference
count.  When the reference count goes down to zero, `SafeRefCounted`
will call `destroy` against the payload and call `free` to
deallocate the store.  If the `T` payload contains any references
to GC-allocated memory, then `SafeRefCounted` will add it to the GC memory
that is scanned for pointers, and remove it from GC scanning before
`free` is called on the store.

One important consequence of `destroy` is that it will call the
destructor of the `T` payload.  GC-managed references are not
guaranteed to be valid during a destructor call, but other members of
`T`, such as file handles or pointers to `malloc` memory, will
still be valid during the destructor call.  This allows the `T` to
deallocate or clean up any non-GC resources immediately after the
reference count has reached zero.

Without -preview=dip1000, `SafeRefCounted` is unsafe and should be
used with care. No references to the payload should be escaped outside
the `SafeRefCounted` object.

With -preview=dip1000, `SafeRefCounted` is safe if it's payload is accessed only
with the $(LREF borrow) function. Scope semantics can also prevent accidental
escaping of `refCountedPayload`, but it's still up to the user to not destroy
the last counted reference while the payload is in use. Due to that,
`refCountedPayload` remains accessible only in `@system` code.

The `autoInit` option makes the object ensure the store is
automatically initialized. Leaving $(D autoInit ==
RefCountedAutoInitialize.yes) (the default option) is convenient but
has the cost of a test whenever the payload is accessed. If $(D
autoInit == RefCountedAutoInitialize.no), user code must call either
`refCountedStore.isInitialized` or `refCountedStore.ensureInitialized`
before attempting to access the payload. Not doing so results in null
pointer dereference.

If `T.this()` is annotated with `@disable` then `autoInit` must be
`RefCountedAutoInitialize.no` in order to compile.

See_Also:
  $(LREF RefCounted)
 */
struct SafeRefCounted(T, RefCountedAutoInitialize autoInit =
        RefCountedAutoInitialize.yes)
if (!is(T == class) && !(is(T == interface)))
{
    version (D_BetterC)
    {
        private enum enableGCScan = false;
    }
    else
    {
        private enum enableGCScan = hasIndirections!T;
    }

    extern(C) private pure nothrow @nogc static
    {
        pragma(mangle, "free") void pureFree( void *ptr );
        static if (enableGCScan)
            import core.memory : GC;
    }

    pragma(inline, true) private void checkInit()()
    if (autoInit == RefCountedAutoInitialize.yes)
    {
        _refCounted.ensureInitialized();
    }

    pragma(inline, true) private void checkInit()() inout
    if (autoInit == RefCountedAutoInitialize.no)
    {
        assert(_refCounted.isInitialized,
            "Attempted to use an uninitialized payload.");
    }

    /// `SafeRefCounted` storage implementation.
    struct RefCountedStore
    {
        private struct Impl
        {
            T _payload;
            size_t _count;
        }

        private Impl* _store;

        private void initialize(A...)(auto ref A args)
        {
            import core.lifetime : emplace, forward;

            allocateStore();
            version (D_Exceptions) scope(failure) () @trusted { deallocateStore(); }();
            emplace(&_store._payload, forward!args);
            _store._count = 1;
        }

        private void move(ref T source) nothrow pure
        {
            import std.algorithm.mutation : moveEmplace;

            allocateStore();
            () @trusted { moveEmplace(source, _store._payload); }();
            _store._count = 1;
        }

        // 'nothrow': can only generate an Error
        private void allocateStore() nothrow pure
        {
            static if (enableGCScan)
            {
                import std.internal.memory : enforceCalloc;
                auto ptr = enforceCalloc(1, Impl.sizeof);
                _store = () @trusted { return cast(Impl*) ptr; }();
                () @trusted { GC.addRange(&_store._payload, T.sizeof); }();
            }
            else
            {
                import std.internal.memory : enforceMalloc;
                auto ptr = enforceMalloc(Impl.sizeof);
                _store = () @trusted { return cast(Impl*) ptr; }();
            }
        }

        private void deallocateStore() nothrow pure
        {
            static if (enableGCScan)
            {
                GC.removeRange(&this._store._payload);
            }
            pureFree(_store);
            _store = null;
        }

        /**
           Returns `true` if and only if the underlying store has been
           allocated and initialized.
        */
        @property nothrow @safe pure @nogc
        bool isInitialized() const
        {
            return _store !is null;
        }

        /**
           Returns underlying reference count if it is allocated and initialized
           (a positive integer), and `0` otherwise.
        */
        @property nothrow @safe pure @nogc
        size_t refCount() const
        {
            return isInitialized ? _store._count : 0;
        }

        /**
           Makes sure the payload was properly initialized. Such a
           call is typically inserted before using the payload.

           This function is unavailable if `T.this()` is annotated with
           `@disable`.
        */
        @safe pure nothrow
        void ensureInitialized()()
        {
            // By checking for `@disable this()` and failing early we can
            // produce a clearer error message.
            static assert(__traits(compiles, { static T t; }),
                "Cannot automatically initialize `" ~ fullyQualifiedName!T ~
                "` because `" ~ fullyQualifiedName!T ~
                ".this()` is annotated with `@disable`.");
            if (!isInitialized) initialize();
        }

    }
    RefCountedStore _refCounted;

    /// Returns storage implementation struct.
    @property nothrow @safe
    ref inout(RefCountedStore) refCountedStore() inout
    {
        return _refCounted;
    }

/**
Constructor that initializes the payload.

Postcondition: `refCountedStore.isInitialized`
 */
    this(A...)(auto ref A args)
    if (A.length > 0)
    out
    {
        assert(refCountedStore.isInitialized);
    }
    do
    {
        import core.lifetime : forward;
        _refCounted.initialize(forward!args);
    }

    /// Ditto
    this(return scope T val)
    {
        _refCounted.move(val);
    }

/**
Constructor that tracks the reference count appropriately. If $(D
!refCountedStore.isInitialized), does nothing.
 */
    this(this) @safe pure nothrow @nogc
    {
        if (!_refCounted.isInitialized) return;
        ++_refCounted._store._count;
    }

/**
Destructor that tracks the reference count appropriately. If $(D
!refCountedStore.isInitialized), does nothing. When the reference count goes
down to zero, calls `destroy` agaist the payload and calls `free`
to deallocate the corresponding resource.
 */
    ~this()
    {
        import std.traits : dip1000Enabled;

        // This prevents the same reference from decrementing the count twice.
        scope(exit) _refCounted = _refCounted.init;

        if (!_refCounted.isInitialized) return;
        assert(_refCounted._store._count > 0);
        if (--_refCounted._store._count) return;
        // Done, destroy and deallocate
        .destroy(_refCounted._store._payload);

        static if (dip1000Enabled)
        {
            () @trusted { _refCounted.deallocateStore(); }();
        }
        else _refCounted.deallocateStore();
    }

/**
Assignment operators.

Note: You may not assign a new payload to an uninitialized SafeRefCounted, if
auto initialization is off. Assigning another counted reference is still okay.
*/
    void opAssign(typeof(this) rhs)
    {
        import std.algorithm.mutation : swap;

        swap(_refCounted._store, rhs._refCounted._store);
    }

/// Ditto
    void opAssign(T rhs)
    {
        import std.algorithm.mutation : move;

        checkInit();
        move(rhs, _refCounted._store._payload);
    }

    //version to have a single properly ddoc'ed function (w/ correct sig)
    version (StdDdoc)
    {
        /**
        Returns a reference to the payload. If (autoInit ==
        RefCountedAutoInitialize.yes), calls $(D
        refCountedStore.ensureInitialized). Otherwise, just issues $(D
        assert(refCountedStore.isInitialized)). Used with $(D alias
        refCountedPayload this;), so callers can just use the `SafeRefCounted`
        object as a `T`.

        $(BLUE The first overload exists only if $(D autoInit == RefCountedAutoInitialize.yes).)
        So if $(D autoInit == RefCountedAutoInitialize.no)
        or called for a constant or immutable object, then
        `refCountedPayload` will also be qualified as nothrow
        (but will still assert if not initialized).
         */
        @property @system
        ref T refCountedPayload() return;

        /// ditto
        @property nothrow @system pure @nogc
        ref inout(T) refCountedPayload() inout return;
    }
    else
    {
        static if (autoInit == RefCountedAutoInitialize.yes)
        {
            //Can't use inout here because of potential mutation
            @property @system
            ref T refCountedPayload() return
            {
                checkInit();
                return _refCounted._store._payload;
            }
        }
        else
        {
            @property nothrow @system pure @nogc
            ref inout(T) refCountedPayload() inout return
            {
                checkInit();
                return _refCounted._store._payload;
            }
        }
    }

/**
Returns a reference to the payload. If (autoInit ==
RefCountedAutoInitialize.yes), calls $(D
refCountedStore.ensureInitialized). Otherwise, just issues $(D
assert(refCountedStore.isInitialized)).
 */
    alias refCountedPayload this;

    static if (is(T == struct) && !is(typeof((ref T t) => t.toString())))
    {
        string toString(this This)()
        {
            import std.conv : to;

            static if (autoInit)
                return to!string(refCountedPayload);
            else
            {
                if (!_refCounted.isInitialized)
                    return This.stringof ~ "(RefCountedStore(null))";
                else
                    return to!string(_refCounted._store._payload);
            }
        }
    }
}

///
@betterC pure @system nothrow @nogc unittest
{
    // A pair of an `int` and a `size_t` - the latter being the
    // reference count - will be dynamically allocated
    auto rc1 = SafeRefCounted!int(5);
    assert(rc1 == 5);
    // No more allocation, add just one extra reference count
    auto rc2 = rc1;
    // Reference semantics
    rc2 = 42;
    assert(rc1 == 42);
    // the pair will be freed when rc1 and rc2 go out of scope
}

// This test can't be betterC because the test extractor won't see the private
// `initialize` method accessed here
pure @safe nothrow @nogc unittest
{
    auto rc1 = SafeRefCounted!(int, RefCountedAutoInitialize.no)(5);
    rc1._refCounted.initialize();
}

pure @system unittest
{
    foreach (MyRefCounted; AliasSeq!(SafeRefCounted, RefCounted))
    {
        MyRefCounted!int* p;
        auto rc1 = MyRefCounted!int(5);
        p = &rc1;
        assert(rc1 == 5);
        assert(rc1._refCounted._store._count == 1);
        {
            auto rc2 = rc1;
            assert(rc1._refCounted._store._count == 2);
            // Reference semantics
            rc2 = 42;
            assert(rc1 == 42);
            rc2 = rc2;
            assert(rc2._refCounted._store._count == 2);
            rc1 = rc2;
            assert(rc1._refCounted._store._count == 2);
        }
        // Artificially end scope of rc1 by calling ~this() explicitly
        rc1.__xdtor();
        assert(p._refCounted._store == null);

        // [Safe]RefCounted as a member
        struct A
        {
            MyRefCounted!int x;
            this(int y)
            {
                x._refCounted.initialize(y);
            }
            A copy()
            {
                auto another = this;
                return another;
            }
        }
        auto a = A(4);
        auto b = a.copy();
        assert(a.x._refCounted._store._count == 2,
               "https://issues.dlang.org/show_bug.cgi?id=4356 still unfixed");
   }
}

@betterC pure @safe nothrow @nogc unittest
{
    import std.algorithm.mutation : swap;

    SafeRefCounted!int p1, p2;
    swap(p1, p2);
}

// Same as above but for old RefCounted and not @safe
@betterC pure @system nothrow @nogc unittest
{
    import std.algorithm.mutation : swap;

    RefCounted!int p1, p2;
    swap(p1, p2);
}

// https://issues.dlang.org/show_bug.cgi?id=6606
@betterC @safe pure nothrow @nogc unittest
{
    union U {
       size_t i;
       void* p;
    }

    struct S {
       U u;
    }

    alias SRC = SafeRefCounted!S;
}

// Same as above but for old RefCounted and not @safe
@betterC @system pure nothrow @nogc unittest
{
    union U {
       size_t i;
       void* p;
    }

    struct S {
       U u;
    }

    alias SRC = RefCounted!S;
}

// https://issues.dlang.org/show_bug.cgi?id=6436
@betterC @system pure unittest
{
    import std.meta : AliasSeq;
    struct S
    {
        this(int rval) { assert(rval == 1); }
        this(ref int lval) { assert(lval == 3); ++lval; }
    }

    foreach (MyRefCounted; AliasSeq!(SafeRefCounted, RefCounted))
    {
        auto s1 = MyRefCounted!S(1);
        int lval = 3;
        auto s2 = MyRefCounted!S(lval);
        assert(lval == 4);
    }
}

// gc_addRange coverage
@betterC @safe pure unittest
{
    struct S { int* p; }

    auto s = SafeRefCounted!S(null);
}

// Same as above but for old RefCounted and not @safe
@betterC @system pure unittest
{
    struct S { int* p; }

    auto s = RefCounted!S(null);
}

@betterC @system pure nothrow @nogc unittest
{
    import std.meta : AliasSeq;
    foreach (MyRefCounted; AliasSeq!(SafeRefCounted, RefCounted))
    {
        MyRefCounted!int a;
        a = 5; //This should not assert
        assert(a == 5);

        MyRefCounted!int b;
        b = a; //This should not assert either
        assert(b == 5);

        MyRefCounted!(int*) c;
    }
}

// https://issues.dlang.org/show_bug.cgi?id=21638
@betterC @system pure nothrow @nogc unittest
{
    import std.meta : AliasSeq;
    static struct NoDefaultCtor
    {
        @disable this();
        this(int x) @nogc nothrow pure { this.x = x; }
        int x;
    }

    foreach (MyRefCounted; AliasSeq!(SafeRefCounted, RefCounted))
    {
        auto rc = MyRefCounted!(NoDefaultCtor, RefCountedAutoInitialize.no)(5);
        assert(rc.x == 5);
    }
}

// https://issues.dlang.org/show_bug.cgi?id=20502
@system unittest
{
    alias Types = AliasSeq!(SafeRefCounted, RefCounted);
    alias funcs = AliasSeq!(safeRefCounted, refCounted);
    static foreach (aliasI; 0 .. 2)
    {{
        alias MyRefCounted = Types[aliasI];
        alias myRefCounted = funcs[aliasI];
        import std.conv : to;

        // Check that string conversion is transparent for refcounted
        // structs that do not have either toString or alias this.
        static struct A { Object a; }
        auto a  = A(new Object());
        auto r = myRefCounted(a);
        assert(to!string(r) == to!string(a));
        assert(to!string(cast(const) r) == to!string(cast(const) a));
        // Check that string conversion is still transparent for refcounted
        // structs that have alias this.
        static struct B { int b; alias b this; }
        static struct C { B b; alias b this; }
        assert(to!string(myRefCounted(C(B(123)))) == to!string(C(B(123))));
        // https://issues.dlang.org/show_bug.cgi?id=22093
        // Check that uninitialized refcounted structs that previously could be
        // converted to strings still can be.
        alias R = typeof(r);
        R r2;
        cast(void) (((const ref R a) => to!string(a))(r2));
        cast(void) to!string(MyRefCounted!(A, RefCountedAutoInitialize.no).init);
    }}
}

// We tried to make `refCountedPayload` `@safe` in
// https://github.com/dlang/phobos/pull/8368 . It proved impossible, but it may
// be possible in the future. This test checks for false `@safe` issues we
// encountered.
@safe unittest
{
    struct Container
    {
        int[] data;
    }

    int[] getArr1 (scope Container local)
    {
        // allowed because the argument is inferred as return scope.
        return local.data;
    }

    int[] getArr2 (scope Container local)
    {
        SafeRefCounted!Container rc = local;
        // Escapes a reference to expired reference counted struct
        // don't do this!
        return rc.refCountedPayload().data;
    }

    int destroyFirstAndUseLater()
    {
        auto rc = SafeRefCounted!int(123);
        int* ptr = &rc.refCountedPayload();
        destroy(rc);
        return *ptr;
    }

    // This is here mainly to test that safety gets inferred correctly for the
    // next tests
    static assert(isSafe!getArr1);
    // https://github.com/dlang/phobos/pull/8101#issuecomment-843017282
    // This got apparently fixed automatically by compiler updates.
    static assert(!isSafe!getArr2);
    // As of writing, this is the issue that is still preventing payload access
    // from being `@safe`
    static assert(!isSafe!destroyFirstAndUseLater);
}

/**
Borrows the payload of $(LREF SafeRefCounted) for use in `fun`. Inferred as `@safe`
if `fun` is `@safe` and does not escape a reference to the payload.
The reference count will be incremented for the duration of the operation,
so destroying the last reference will not leave dangling references in
`fun`.

Params:
  fun = A callable accepting the payload either by value or by reference.
  refCount = The counted reference to the payload.
Returns:
  The return value of `fun`, if any. `ref` in the return value will be
  forwarded.
Issues:
  For yet unknown reason, code that uses this function with UFCS syntax
  will not be inferred as `@safe`. It will still compile if the code is
  explicitly marked `@safe` and nothing in `fun` prevents that.
*/
template borrow(alias fun)
{
    import std.functional : unaryFun;

    auto ref borrow(RC)(RC refCount)
    if
    (
        isInstanceOf!(SafeRefCounted, RC)
        && is(typeof(unaryFun!fun(refCount.refCountedPayload)))
    )
    {
        refCount.checkInit();

        // If `fun` escapes a reference to the payload, it will be inferred
        // as unsafe due to the scope storage class here.
        scope plPtr = &refCount._refCounted._store._payload;
        return unaryFun!fun(*plPtr);

        // We destroy our copy of the reference here, automatically destroying
        // the payload if `fun` destroyed the last reference outside.
    }
}

/// This example can be marked `@safe` with `-preview=dip1000`.
@safe pure nothrow unittest
{
    auto rcInt = safeRefCounted(5);
    assert(rcInt.borrow!(theInt => theInt) == 5);
    auto sameInt = rcInt;
    assert(sameInt.borrow!"a" == 5);

    // using `ref` in the function
    auto arr = [0, 1, 2, 3, 4, 5, 6];
    sameInt.borrow!(ref (x) => arr[x]) = 10;
    assert(arr == [0, 1, 2, 3, 4, 10, 6]);

    // modifying the payload via an alias
    sameInt.borrow!"a*=2";
    assert(rcInt.borrow!"a" == 10);
}

// Some memory safety penetration testing.
@system unittest
{
    int* globalPtr;
    int torpedoesFired = 0;
    struct Destroyer { ~this() @safe { torpedoesFired++; } }

    alias RcInt = typeof(safeRefCounted(0));
    auto standardUsage(RcInt arg)
    {
        return borrow!((ref x) => x)(arg);
    }
    ref harmlessRefReturn(RcInt arg)
    {
        return borrow!(ref (ref x) => *globalPtr = x)(arg);
    }
    ref problematicRefReturn(RcInt arg)
    {
        return borrow!(ref (ref x) => x)(arg);
    }
    auto sideChannelEscape(RcInt arg)
    {
        return borrow!((ref x)
        {
            globalPtr = &x;
            return x;
        })(arg);
    }
    auto destroyDuringApply()
    {
        auto rc = safeRefCounted(Destroyer());
        return borrow!((ref x)
        {
            // Destroys the last reference to the payload, decrementing it's
            // reference count.
            rc.__dtor();
            // Destroy again! rc should be set to it's init value so that this
            // won't decrement the reference count of the original payload.
            rc.__dtor();
            // The internal reference count increment done for duration of
            // `apply` should make sure that the payload destructor is not yet
            // run, and this value thus not incremented.
            return torpedoesFired;
        })(rc);
    }

    // First, let's verify the dangerous functions really do what they are
    // supposed to do.
    auto testRc = safeRefCounted(42);
    assert(sideChannelEscape(testRc) == 42);
    assert(&problematicRefReturn(testRc) == globalPtr);

    // Now, are the @safe attributes inferred correctly?
    assert(isSafe!standardUsage);
    assert(isSafe!harmlessRefReturn);
    assert(!isSafe!problematicRefReturn);
    assert(!isSafe!sideChannelEscape);
    assert(isSafe!destroyDuringApply);

    // Finally, we test protection against early destruction during `apply`.
    auto torpedoesUpToReturn = destroyDuringApply();
    assert(torpedoesFired == torpedoesUpToReturn + 1);
}

/**
 * Initializes a `SafeRefCounted` with `val`. The template parameter
 * `T` of `SafeRefCounted` is inferred from `val`.
 * This function can be used to move non-copyable values to the heap.
 * It also disables the `autoInit` option of `SafeRefCounted`.
 *
 * Params:
 *   val = The value to be reference counted
 * Returns:
 *   An initialized `SafeRefCounted` containing `val`.
 * See_Also:
 *   $(LREF refCounted)
 *   $(HTTP en.cppreference.com/w/cpp/memory/shared_ptr/make_shared, C++'s make_shared)
 */
SafeRefCounted!(T, RefCountedAutoInitialize.no) safeRefCounted(T)(T val)
{
    typeof(return) res;
    res._refCounted.move(val);
    return res;
}

///
@system unittest
{
    static struct File
    {
        static size_t nDestroyed;
        string name;
        @disable this(this); // not copyable
        ~this() { name = null; ++nDestroyed; }
    }

    auto file = File("name");
    assert(file.name == "name");
    // file cannot be copied and has unique ownership
    static assert(!__traits(compiles, {auto file2 = file;}));

    assert(File.nDestroyed == 0);

    // make the file ref counted to share ownership
    // Note:
    //   We write a compound statement (brace-delimited scope) in which all `SafeRefCounted!File` handles are created and deleted.
    //   This allows us to see (after the scope) what happens after all handles have been destroyed.
    {
        // We move the content of `file` to a separate (and heap-allocated) `File` object,
        // managed-and-accessed via one-or-multiple (initially: one) `SafeRefCounted!File` objects ("handles").
        // This "moving":
        //   (1) invokes `file`'s destructor (=> `File.nDestroyed` is incremented from 0 to 1 and `file.name` becomes `null`);
        //   (2) overwrites `file` with `File.init` (=> `file.name` becomes `null`).
        // It appears that writing `name = null;` in the destructor is redundant,
        // but please note that (2) is only performed if `File` defines a destructor (or post-blit operator),
        // and in the absence of the `nDestroyed` instrumentation there would have been no reason to define a destructor.
        import std.algorithm.mutation : move;
        auto rcFile = safeRefCounted(move(file));
        assert(rcFile.name == "name");
        assert(File.nDestroyed == 1);
        assert(file.name == null);

        // We create another `SafeRefCounted!File` handle to the same separate `File` object.
        // While any of the handles is still alive, the `File` object is kept alive (=> `File.nDestroyed` is not modified).
        auto rcFile2 = rcFile;
        assert(rcFile.refCountedStore.refCount == 2);
        assert(File.nDestroyed == 1);
    }
    // The separate `File` object is deleted when the last `SafeRefCounted!File` handle is destroyed
    // (i.e. at the closing brace of the compound statement above, which destroys both handles: `rcFile` and `rcFile2`)
    // (=> `File.nDestroyed` is incremented again, from 1 to 2):
    assert(File.nDestroyed == 2);
}

/**
    Creates a proxy for the value `a` that will forward all operations
    while disabling implicit conversions. The aliased item `a` must be
    an $(B lvalue). This is useful for creating a new type from the
    "base" type (though this is $(B not) a subtype-supertype
    relationship; the new type is not related to the old type in any way,
    by design).

    The new type supports all operations that the underlying type does,
    including all operators such as `+`, `--`, `<`, `[]`, etc.

    Params:
        a = The value to act as a proxy for all operations. It must
            be an lvalue.
 */
mixin template Proxy(alias a)
{
    private alias ValueType = typeof({ return a; }());

    /* Determine if 'T.a' can referenced via a const(T).
     * Use T* as the parameter because 'scope' inference needs a fully
     * analyzed T, which doesn't work when accessibleFrom() is used in a
     * 'static if' in the definition of Proxy or T.
     */
    private enum bool accessibleFrom(T) =
        is(typeof((T* self){ cast(void) mixin("(*self)."~__traits(identifier, a)); }));

    static if (is(typeof(this) == class))
    {
        override bool opEquals(Object o)
        {
            if (auto b = cast(typeof(this))o)
            {
                return a == mixin("b."~__traits(identifier, a));
            }
            return false;
        }

        bool opEquals(T)(T b)
        if (is(ValueType : T) || is(typeof(a.opEquals(b))) || is(typeof(b.opEquals(a))))
        {
            static if (is(typeof(a.opEquals(b))))
                return a.opEquals(b);
            else static if (is(typeof(b.opEquals(a))))
                return b.opEquals(a);
            else
                return a == b;
        }

        override int opCmp(Object o)
        {
            if (auto b = cast(typeof(this))o)
            {
                return a < mixin("b."~__traits(identifier, a)) ? -1
                     : a > mixin("b."~__traits(identifier, a)) ? +1 : 0;
            }
            static if (is(ValueType == class))
                return a.opCmp(o);
            else
                throw new Exception("Attempt to compare a "~typeid(this).toString~" and a "~typeid(o).toString);
        }

        int opCmp(T)(auto ref const T b)
        if (is(ValueType : T) || is(typeof(a.opCmp(b))) || is(typeof(b.opCmp(a))))
        {
            static if (is(typeof(a.opCmp(b))))
                return a.opCmp(b);
            else static if (is(typeof(b.opCmp(a))))
                return -b.opCmp(a);
            else
                return a < b ? -1 : a > b ? +1 : 0;
        }

        static if (accessibleFrom!(const typeof(this)))
        {
            override size_t toHash() const nothrow @safe
            {
                static if (__traits(compiles, .hashOf(a)))
                    return .hashOf(a);
                else
                // Workaround for when .hashOf is not both @safe and nothrow.
                {
                    static if (is(typeof(&a) == ValueType*))
                        alias v = a;
                    else
                        auto v = a; // if a is (property) function
                    // BUG: Improperly casts away `shared`!
                    return typeid(ValueType).getHash((() @trusted => cast(const void*) &v)());
                }
            }
        }
    }
    else
    {
        auto ref opEquals(this X, B)(auto ref B b)
        {
            static if (is(immutable B == immutable typeof(this)))
            {
                return a == mixin("b."~__traits(identifier, a));
            }
            else
                return a == b;
        }

        auto ref opCmp(this X, B)(auto ref B b)
        {
            static if (is(typeof(a.opCmp(b))))
                return a.opCmp(b);
            else static if (is(typeof(b.opCmp(a))))
                return -b.opCmp(a);
            else static if (isFloatingPoint!ValueType || isFloatingPoint!B)
                return a < b ? -1 : a > b ? +1 : a == b ? 0 : float.nan;
            else
                return a < b ? -1 : (a > b);
        }

        static if (accessibleFrom!(const typeof(this)))
        {
            size_t toHash() const nothrow @safe
            {
                static if (__traits(compiles, .hashOf(a)))
                    return .hashOf(a);
                else
                // Workaround for when .hashOf is not both @safe and nothrow.
                {
                    static if (is(typeof(&a) == ValueType*))
                        alias v = a;
                    else
                        auto v = a; // if a is (property) function
                    // BUG: Improperly casts away `shared`!
                    return typeid(ValueType).getHash((() @trusted => cast(const void*) &v)());
                }
            }
        }
    }

    auto ref opCall(this X, Args...)(auto ref Args args) { return a(args); }

    auto ref opCast(T, this X)() { return cast(T) a; }

    auto ref opIndex(this X, D...)(auto ref D i)               { return a[i]; }
    auto ref opSlice(this X      )()                           { return a[]; }
    auto ref opSlice(this X, B, E)(auto ref B b, auto ref E e) { return a[b .. e]; }

    auto ref opUnary     (string op, this X      )()                           { return mixin(op~"a"); }
    auto ref opIndexUnary(string op, this X, D...)(auto ref D i)               { return mixin(op~"a[i]"); }
    auto ref opSliceUnary(string op, this X      )()                           { return mixin(op~"a[]"); }
    auto ref opSliceUnary(string op, this X, B, E)(auto ref B b, auto ref E e) { return mixin(op~"a[b .. e]"); }

    auto ref opBinary(string op, this X, B)(auto ref B b)
    if (op == "in" && is(typeof(a in b)) || op != "in")
    {
        return mixin("a "~op~" b");
    }
    auto ref opBinaryRight(string op, this X, B)(auto ref B b) { return mixin("b "~op~" a"); }

    static if (!is(typeof(this) == class))
    {
        import std.traits;
        static if (isAssignable!ValueType)
        {
            auto ref opAssign(this X)(auto ref typeof(this) v)
            {
                a = mixin("v."~__traits(identifier, a));
                return this;
            }
        }
        else
        {
            @disable void opAssign(this X)(auto ref typeof(this) v);
        }
    }

    auto ref opAssign     (this X, V      )(auto ref V v)
    if (!is(V == typeof(this))) { return a       = v; }
    auto ref opIndexAssign(this X, V, D...)(auto ref V v, auto ref D i)               { return a[i]    = v; }
    auto ref opSliceAssign(this X, V      )(auto ref V v)                             { return a[]     = v; }
    auto ref opSliceAssign(this X, V, B, E)(auto ref V v, auto ref B b, auto ref E e) { return a[b .. e] = v; }

    auto ref opOpAssign     (string op, this X, V      )(auto ref V v)
    {
        return mixin("a = a "~op~" v");
    }
    auto ref opIndexOpAssign(string op, this X, V, D...)(auto ref V v, auto ref D i)
    {
        return mixin("a[i] "   ~op~"= v");
    }
    auto ref opSliceOpAssign(string op, this X, V      )(auto ref V v)
    {
        return mixin("a[] "    ~op~"= v");
    }
    auto ref opSliceOpAssign(string op, this X, V, B, E)(auto ref V v, auto ref B b, auto ref E e)
    {
        return mixin("a[b .. e] "~op~"= v");
    }

    template opDispatch(string name)
    {
        static if (is(typeof(__traits(getMember, a, name)) == function))
        {
            // non template function
            auto ref opDispatch(this X, Args...)(auto ref Args args) { return mixin("a."~name~"(args)"); }
        }
        else static if (is(typeof({ enum x = mixin("a."~name); })))
        {
            // built-in type field, manifest constant, and static non-mutable field
            enum opDispatch = mixin("a."~name);
        }
        else static if (__traits(isTemplate, mixin("a."~name)))
        {
            // member template
            template opDispatch(T...)
            {
                enum targs = T.length ? "!T" : "";
                auto ref opDispatch(this X, Args...)(auto ref Args args){ return mixin("a."~name~targs~"(args)"); }
            }
        }
        else
        {
            // field or property function
            @property auto ref opDispatch(this X)()                { return mixin("a."~name);        }
            @property auto ref opDispatch(this X, V)(auto ref V v) { return mixin("a."~name~" = v"); }
        }

    }

    import std.traits : isArray;

    static if (isArray!ValueType)
    {
        auto opDollar() const { return a.length; }
    }
    else static if (is(typeof(a.opDollar!0)))
    {
        auto ref opDollar(size_t pos)() { return a.opDollar!pos(); }
    }
    else static if (is(typeof(a.opDollar) == function))
    {
        auto ref opDollar() { return a.opDollar(); }
    }
    else static if (is(typeof(a.opDollar)))
    {
        alias opDollar = a.opDollar;
    }
}

///
@safe unittest
{
    struct MyInt
    {
        private int value;
        mixin Proxy!value;

        this(int n){ value = n; }
    }

    MyInt n = 10;

    // Enable operations that original type has.
    ++n;
    assert(n == 11);
    assert(n * 2 == 22);

    void func(int n) { }

    // Disable implicit conversions to original type.
    //int x = n;
    //func(n);
}

///The proxied value must be an $(B lvalue).
@safe unittest
{
    struct NewIntType
    {
        //Won't work; the literal '1'
        //is an rvalue, not an lvalue
        //mixin Proxy!1;

        //Okay, n is an lvalue
        int n;
        mixin Proxy!n;

        this(int n) { this.n = n; }
    }

    NewIntType nit = 0;
    nit++;
    assert(nit == 1);


    struct NewObjectType
    {
        Object obj;
        //Ok, obj is an lvalue
        mixin Proxy!obj;

        this (Object o) { obj = o; }
    }

    NewObjectType not = new Object();
    assert(__traits(compiles, not.toHash()));
}

/**
    There is one exception to the fact that the new type is not related to the
    old type. $(DDSUBLINK spec/function,pseudo-member, Pseudo-member)
    functions are usable with the new type; they will be forwarded on to the
    proxied value.
 */
@safe unittest
{
    import std.math.traits : isInfinity;

    float f = 1.0;
    assert(!f.isInfinity);

    struct NewFloat
    {
        float _;
        mixin Proxy!_;

        this(float f) { _ = f; }
    }

    NewFloat nf = 1.0f;
    assert(!nf.isInfinity);
}

@safe unittest
{
    static struct MyInt
    {
        private int value;
        mixin Proxy!value;
        this(int n) inout { value = n; }

        enum str = "str";
        static immutable arr = [1,2,3];
    }

    static foreach (T; AliasSeq!(MyInt, const MyInt, immutable MyInt))
    {{
        T m = 10;
        static assert(!__traits(compiles, { int x = m; }));
        static assert(!__traits(compiles, { void func(int n){} func(m); }));
        assert(m == 10);
        assert(m != 20);
        assert(m < 20);
        assert(+m == 10);
        assert(-m == -10);
        assert(cast(double) m == 10.0);
        assert(m + 10 == 20);
        assert(m - 5 == 5);
        assert(m * 20 == 200);
        assert(m / 2 == 5);
        assert(10 + m == 20);
        assert(15 - m == 5);
        assert(20 * m == 200);
        assert(50 / m == 5);
        static if (is(T == MyInt))  // mutable
        {
            assert(++m == 11);
            assert(m++ == 11); assert(m == 12);
            assert(--m == 11);
            assert(m-- == 11); assert(m == 10);
            m = m;
            m = 20; assert(m == 20);
        }
        static assert(T.max == int.max);
        static assert(T.min == int.min);
        static assert(T.init == int.init);
        static assert(T.str == "str");
        static assert(T.arr == [1,2,3]);
    }}
}
@system unittest
{
    static struct MyArray
    {
        private int[] value;
        mixin Proxy!value;
        this(int[] arr) { value = arr; }
        this(immutable int[] arr) immutable { value = arr; }
    }

    static foreach (T; AliasSeq!(MyArray, const MyArray, immutable MyArray))
    {{
      static if (is(T == immutable) && !is(typeof({ T a = [1,2,3,4]; })))
        T a = [1,2,3,4].idup;   // workaround until qualified ctor is properly supported
      else
        T a = [1,2,3,4];
        assert(a == [1,2,3,4]);
        assert(a != [5,6,7,8]);
        assert(+a[0]    == 1);
        version (LittleEndian)
            assert(cast(ulong[]) a == [0x0000_0002_0000_0001, 0x0000_0004_0000_0003]);
        else
            assert(cast(ulong[]) a == [0x0000_0001_0000_0002, 0x0000_0003_0000_0004]);
        assert(a ~ [10,11] == [1,2,3,4,10,11]);
        assert(a[0]    == 1);
        assert(a[]     == [1,2,3,4]);
        assert(a[2 .. 4] == [3,4]);
        static if (is(T == MyArray))    // mutable
        {
            a = a;
            a = [5,6,7,8];  assert(a == [5,6,7,8]);
            a[0]     = 0;   assert(a == [0,6,7,8]);
            a[]      = 1;   assert(a == [1,1,1,1]);
            a[0 .. 3]  = 2;   assert(a == [2,2,2,1]);
            a[0]    += 2;   assert(a == [4,2,2,1]);
            a[]     *= 2;   assert(a == [8,4,4,2]);
            a[0 .. 2] /= 2;   assert(a == [4,2,4,2]);
        }
    }}
}
@system unittest
{
    class Foo
    {
        int field;

        @property int val1() const { return field; }
        @property void val1(int n) { field = n; }

        @property ref int val2() { return field; }

        int func(int x, int y) const { return x; }
        void func1(ref int a) { a = 9; }

        T ifti1(T)(T t) { return t; }
        void ifti2(Args...)(Args args) { }
        void ifti3(T, Args...)(Args args) { }

        T opCast(T)(){ return T.init; }

        T tempfunc(T)() { return T.init; }
    }
    class Hoge
    {
        Foo foo;
        mixin Proxy!foo;
        this(Foo f) { foo = f; }
    }

    auto h = new Hoge(new Foo());
    int n;

    static assert(!__traits(compiles, { Foo f = h; }));

    // field
    h.field = 1;            // lhs of assign
    n = h.field;            // rhs of assign
    assert(h.field == 1);   // lhs of BinExp
    assert(1 == h.field);   // rhs of BinExp
    assert(n == 1);

    // getter/setter property function
    h.val1 = 4;
    n = h.val1;
    assert(h.val1 == 4);
    assert(4 == h.val1);
    assert(n == 4);

    // ref getter property function
    h.val2 = 8;
    n = h.val2;
    assert(h.val2 == 8);
    assert(8 == h.val2);
    assert(n == 8);

    // member function
    assert(h.func(2,4) == 2);
    h.func1(n);
    assert(n == 9);

    // IFTI
    assert(h.ifti1(4) == 4);
    h.ifti2(4);
    h.ifti3!int(4, 3);

    // https://issues.dlang.org/show_bug.cgi?id=5896 test
    assert(h.opCast!int() == 0);
    assert(cast(int) h == 0);
    const ih = new const Hoge(new Foo());
    static assert(!__traits(compiles, ih.opCast!int()));
    static assert(!__traits(compiles, cast(int) ih));

    // template member function
    assert(h.tempfunc!int() == 0);
}

@system unittest // about Proxy inside a class
{
    class MyClass
    {
        int payload;
        mixin Proxy!payload;
        this(int i){ payload = i; }
        string opCall(string msg){ return msg; }
        int pow(int i){ return payload ^^ i; }
    }

    class MyClass2
    {
        MyClass payload;
        mixin Proxy!payload;
        this(int i){ payload = new MyClass(i); }
    }

    class MyClass3
    {
        int payload;
        mixin Proxy!payload;
        this(int i){ payload = i; }
    }

    // opEquals
    Object a = new MyClass(5);
    Object b = new MyClass(5);
    Object c = new MyClass2(5);
    Object d = new MyClass3(5);
    assert(a == b);
    assert((cast(MyClass) a) == 5);
    assert(5 == (cast(MyClass) b));
    assert(5 == cast(MyClass2) c);
    assert(a != d);

    assert(c != a);
    // oops! above line is unexpected, isn't it?
    // the reason is below.
    // MyClass2.opEquals knows MyClass but,
    // MyClass.opEquals doesn't know MyClass2.
    // so, c.opEquals(a) is true, but a.opEquals(c) is false.
    // furthermore, opEquals(T) couldn't be invoked.
    assert((cast(MyClass2) c) != (cast(MyClass) a));

    // opCmp
    Object e = new MyClass2(7);
    assert(a < cast(MyClass2) e); // OK. and
    assert(e > a); // OK, but...
    // assert(a < e); // RUNTIME ERROR!
    // assert((cast(MyClass) a) < e); // RUNTIME ERROR!
    assert(3 < cast(MyClass) a);
    assert((cast(MyClass2) e) < 11);

    // opCall
    assert((cast(MyClass2) e)("hello") == "hello");

    // opCast
    assert((cast(MyClass)(cast(MyClass2) c)) == a);
    assert((cast(int)(cast(MyClass2) c)) == 5);

    // opIndex
    class MyClass4
    {
        string payload;
        mixin Proxy!payload;
        this(string s){ payload = s; }
    }
    class MyClass5
    {
        MyClass4 payload;
        mixin Proxy!payload;
        this(string s){ payload = new MyClass4(s); }
    }
    auto f = new MyClass4("hello");
    assert(f[1] == 'e');
    auto g = new MyClass5("hello");
    assert(f[1] == 'e');

    // opSlice
    assert(f[2 .. 4] == "ll");

    // opUnary
    assert(-(cast(MyClass2) c) == -5);

    // opBinary
    assert((cast(MyClass) a) + (cast(MyClass2) c) == 10);
    assert(5 + cast(MyClass) a == 10);

    // opAssign
    (cast(MyClass2) c) = 11;
    assert((cast(MyClass2) c) == 11);
    (cast(MyClass2) c) = new MyClass(13);
    assert((cast(MyClass2) c) == 13);

    // opOpAssign
    assert((cast(MyClass2) c) += 4);
    assert((cast(MyClass2) c) == 17);

    // opDispatch
    assert((cast(MyClass2) c).pow(2) == 289);

    // opDollar
    assert(f[2..$-1] == "ll");

    // toHash
    int[Object] hash;
    hash[a] = 19;
    hash[c] = 21;
    assert(hash[b] == 19);
    assert(hash[c] == 21);
}

@safe unittest
{
    struct MyInt
    {
        int payload;

        mixin Proxy!payload;
    }

    MyInt v;
    v = v;

    struct Foo
    {
        @disable void opAssign(typeof(this));
    }
    struct MyFoo
    {
        Foo payload;

        mixin Proxy!payload;
    }
    MyFoo f;
    static assert(!__traits(compiles, f = f));

    struct MyFoo2
    {
        Foo payload;

        mixin Proxy!payload;

        // override default Proxy behavior
        void opAssign(typeof(this) rhs){}
    }
    MyFoo2 f2;
    f2 = f2;
}

// https://issues.dlang.org/show_bug.cgi?id=8613
@safe unittest
{
    static struct Name
    {
        mixin Proxy!val;
        private string val;
        this(string s) { val = s; }
    }

    bool[Name] names;
    names[Name("a")] = true;
    bool* b = Name("a") in names;
}

// workaround for https://issues.dlang.org/show_bug.cgi?id=19669
private enum isDIP1000 = __traits(compiles, () @safe {
     int x;
     int* p;
     p = &x;
});
// excludes struct S; it's 'mixin Proxy!foo' doesn't compile with -dip1000
static if (isDIP1000) {} else
@system unittest
{
    // https://issues.dlang.org/show_bug.cgi?id=14213
    // using function for the payload
    static struct S
    {
        int foo() { return 12; }
        mixin Proxy!foo;
    }
    S s;
    assert(s + 1 == 13);
    assert(s * 2 == 24);
}

@system unittest
{
    static class C
    {
        int foo() { return 12; }
        mixin Proxy!foo;
    }
    C c = new C();
}

// Check all floating point comparisons for both Proxy and Typedef,
// also against int and a Typedef!int, to be as regression-proof
// as possible. https://issues.dlang.org/show_bug.cgi?id=15561
@safe unittest
{
    static struct MyFloatImpl
    {
        float value;
        mixin Proxy!value;
    }
    static void allFail(T0, T1)(T0 a, T1 b)
    {
        assert(!(a == b));
        assert(!(a<b));
        assert(!(a <= b));
        assert(!(a>b));
        assert(!(a >= b));
    }
    static foreach (T1; AliasSeq!(MyFloatImpl, Typedef!float, Typedef!double,
        float, real, Typedef!int, int))
    {
        static foreach (T2; AliasSeq!(MyFloatImpl, Typedef!float))
        {{
            T1 a;
            T2 b;

            static if (isFloatingPoint!T1 || isFloatingPoint!(TypedefType!T1))
                allFail(a, b);
            a = 3;
            allFail(a, b);

            b = 4;
            assert(a != b);
            assert(a<b);
            assert(a <= b);
            assert(!(a>b));
            assert(!(a >= b));

            a = 4;
            assert(a == b);
            assert(!(a<b));
            assert(a <= b);
            assert(!(a>b));
            assert(a >= b);
        }}
    }
}

/**
$(B Typedef) allows the creation of a unique type which is
based on an existing type. Unlike the `alias` feature,
$(B Typedef) ensures the two types are not considered as equals.

Params:

    init = Optional initial value for the new type.
    cookie = Optional, used to create multiple unique types which are
             based on the same origin type `T`

Note: If a library routine cannot handle the Typedef type,
you can use the `TypedefType` template to extract the
type which the Typedef wraps.
 */
struct Typedef(T, T init = T.init, string cookie=null)
{
    private T Typedef_payload = init;

    // https://issues.dlang.org/show_bug.cgi?id=18415
    // prevent default construction if original type does too.
    static if ((is(T == struct) || is(T == union)) && !is(typeof({T t;})))
    {
        @disable this();
    }

    this(T init)
    {
        Typedef_payload = init;
    }

    this(Typedef tdef)
    {
        this(tdef.Typedef_payload);
    }

    // We need to add special overload for cast(Typedef!X) exp,
    // thus we can't simply inherit Proxy!Typedef_payload
    T2 opCast(T2 : Typedef!(T, Unused), this X, T, Unused...)()
    {
        return T2(cast(T) Typedef_payload);
    }

    auto ref opCast(T2, this X)()
    {
        return cast(T2) Typedef_payload;
    }

    mixin Proxy!Typedef_payload;

    pure nothrow @nogc @safe @property
    {
        alias TD = typeof(this);
        static if (isIntegral!T)
        {
            static TD min() {return TD(T.min);}
            static TD max() {return TD(T.max);}
        }
        else static if (isFloatingPoint!T)
        {
            static TD infinity() {return TD(T.infinity);}
            static TD nan() {return TD(T.nan);}
            static TD dig() {return TD(T.dig);}
            static TD epsilon() {return TD(T.epsilon);}
            static TD mant_dig() {return TD(T.mant_dig);}
            static TD max_10_exp() {return TD(T.max_10_exp);}
            static TD max_exp()  {return TD(T.max_exp);}
            static TD min_10_exp() {return TD(T.min_10_exp);}
            static TD min_exp() {return TD(T.min_exp);}
            static TD max() {return TD(T.max);}
            static TD min_normal() {return TD(T.min_normal);}
            TD re() {return TD(Typedef_payload.re);}
            TD im() {return TD(Typedef_payload.im);}
        }
    }

    /**
     * Convert wrapped value to a human readable string
     */
    string toString(this T)()
    {
        import std.array : appender;
        auto app = appender!string();
        auto spec = singleSpec("%s");
        toString(app, spec);
        return app.data;
    }

    /// ditto
    void toString(this T, W)(ref W writer, scope const ref FormatSpec!char fmt)
    if (isOutputRange!(W, char))
    {
        formatValue(writer, Typedef_payload, fmt);
    }

    ///
    @safe unittest
    {
        import std.conv : to;

        int i = 123;
        auto td = Typedef!int(i);
        assert(i.to!string == td.to!string);
    }
}

///
@safe unittest
{
    alias MyInt = Typedef!int;
    MyInt foo = 10;
    foo++;
    assert(foo == 11);
}

/// custom initialization values
@safe unittest
{
    alias MyIntInit = Typedef!(int, 42);
    static assert(is(TypedefType!MyIntInit == int));
    static assert(MyIntInit() == 42);
}

/// Typedef creates a new type
@safe unittest
{
    alias MyInt = Typedef!int;
    static void takeInt(int) {}
    static void takeMyInt(MyInt) {}

    int i;
    takeInt(i);    // ok
    static assert(!__traits(compiles, takeMyInt(i)));

    MyInt myInt;
    static assert(!__traits(compiles, takeInt(myInt)));
    takeMyInt(myInt);  // ok
}

/// Use the optional `cookie` argument to create different types of the same base type
@safe unittest
{
    alias TypeInt1 = Typedef!int;
    alias TypeInt2 = Typedef!int;

    // The two Typedefs are the same type.
    static assert(is(TypeInt1 == TypeInt2));

    alias MoneyEuros = Typedef!(float, float.init, "euros");
    alias MoneyDollars = Typedef!(float, float.init, "dollars");

    // The two Typedefs are _not_ the same type.
    static assert(!is(MoneyEuros == MoneyDollars));
}

// https://issues.dlang.org/show_bug.cgi?id=12461
@safe unittest
{
    alias Int = Typedef!int;

    Int a, b;
    a += b;
    assert(a == 0);
}

/**
Get the underlying type which a `Typedef` wraps.
If `T` is not a `Typedef` it will alias itself to `T`.
*/
template TypedefType(T)
{
    static if (is(T : Typedef!Arg, Arg))
        alias TypedefType = Arg;
    else
        alias TypedefType = T;
}

///
@safe unittest
{
    import std.conv : to;

    alias MyInt = Typedef!int;
    static assert(is(TypedefType!MyInt == int));

    /// Instantiating with a non-Typedef will return that type
    static assert(is(TypedefType!int == int));

    string num = "5";

    // extract the needed type
    MyInt myInt = MyInt( num.to!(TypedefType!MyInt) );
    assert(myInt == 5);

    // cast to the underlying type to get the value that's being wrapped
    int x = cast(TypedefType!MyInt) myInt;

    alias MyIntInit = Typedef!(int, 42);
    static assert(is(TypedefType!MyIntInit == int));
    static assert(MyIntInit() == 42);
}

@safe unittest
{
    Typedef!int x = 10;
    static assert(!__traits(compiles, { int y = x; }));
    static assert(!__traits(compiles, { long z = x; }));

    Typedef!int y = 10;
    assert(x == y);

    static assert(Typedef!int.init == int.init);

    Typedef!(float, 1.0) z; // specifies the init
    assert(z == 1.0);

    static assert(typeof(z).init == 1.0);

    alias Dollar = Typedef!(int, 0, "dollar");
    alias Yen    = Typedef!(int, 0, "yen");
    static assert(!is(Dollar == Yen));

    Typedef!(int[3]) sa;
    static assert(sa.length == 3);
    static assert(typeof(sa).length == 3);

    Typedef!(int[3]) dollar1;
    assert(dollar1[0..$] is dollar1[0 .. 3]);

    Typedef!(int[]) dollar2;
    dollar2.length = 3;
    assert(dollar2[0..$] is dollar2[0 .. 3]);

    static struct Dollar1
    {
        static struct DollarToken {}
        enum opDollar = DollarToken.init;
        auto opSlice(size_t, DollarToken) { return 1; }
        auto opSlice(size_t, size_t) { return 2; }
    }

    Typedef!Dollar1 drange1;
    assert(drange1[0..$] == 1);
    assert(drange1[0 .. 1] == 2);

    static struct Dollar2
    {
        size_t opDollar(size_t pos)() { return pos == 0 ? 1 : 100; }
        size_t opIndex(size_t i, size_t j) { return i + j; }
    }

    Typedef!Dollar2 drange2;
    assert(drange2[$, $] == 101);

    static struct Dollar3
    {
        size_t opDollar() { return 123; }
        size_t opIndex(size_t i) { return i; }
    }

    Typedef!Dollar3 drange3;
    assert(drange3[$] == 123);
}

// https://issues.dlang.org/show_bug.cgi?id=18415
@safe @nogc pure nothrow unittest
{
    struct NoDefCtorS{@disable this();}
    union NoDefCtorU{@disable this();}
    static assert(!is(typeof({Typedef!NoDefCtorS s;})));
    static assert(!is(typeof({Typedef!NoDefCtorU u;})));
}

// https://issues.dlang.org/show_bug.cgi?id=11703
@safe @nogc pure nothrow unittest
{
    alias I = Typedef!int;
    static assert(is(typeof(I.min) == I));
    static assert(is(typeof(I.max) == I));

    alias F = Typedef!double;
    static assert(is(typeof(F.infinity) == F));
    static assert(is(typeof(F.epsilon) == F));

    F f;
    assert(!is(typeof(F.re).stringof == double));
    assert(!is(typeof(F.im).stringof == double));
}

@safe unittest
{
    // https://issues.dlang.org/show_bug.cgi?id=8655
    import std.typecons;
    import std.bitmanip;
    static import core.stdc.config;

    alias c_ulong = Typedef!(core.stdc.config.c_ulong);

    static struct Foo
    {
        mixin(bitfields!(
            c_ulong, "NameOffset", 31,
            c_ulong, "NameIsString", 1
        ));
    }
}

// https://issues.dlang.org/show_bug.cgi?id=12596
@safe unittest
{
    import std.typecons;
    alias TD = Typedef!int;
    TD x = TD(1);
    TD y = TD(x);
    assert(x == y);
}

@safe unittest // about toHash
{
    import std.typecons;
    {
        alias TD = Typedef!int;
        int[TD] td;
        td[TD(1)] = 1;
        assert(td[TD(1)] == 1);
    }

    {
        alias TD = Typedef!(int[]);
        int[TD] td;
        td[TD([1,2,3,4])] = 2;
        assert(td[TD([1,2,3,4])] == 2);
    }

    {
        alias TD = Typedef!(int[][]);
        int[TD] td;
        td[TD([[1,0,0,0], [0,1,0,0], [0,0,1,0], [0,0,0,1]])] = 3;
        assert(td[TD([[1,0,0,0], [0,1,0,0], [0,0,1,0], [0,0,0,1]])] == 3);
    }

    {
        struct MyStruct{ int x; }
        alias TD = Typedef!MyStruct;
        int[TD] td;
        td[TD(MyStruct(10))] = 4;
        assert(TD(MyStruct(20)) !in td);
        assert(td[TD(MyStruct(10))] == 4);
    }

    {
        static struct MyStruct2
        {
            int x;
            size_t toHash() const nothrow @safe { return x; }
            bool opEquals(ref const MyStruct2 r) const { return r.x == x; }
        }

        alias TD = Typedef!MyStruct2;
        int[TD] td;
        td[TD(MyStruct2(50))] = 5;
        assert(td[TD(MyStruct2(50))] == 5);
    }

    {
        class MyClass{}
        alias TD = Typedef!MyClass;
        int[TD] td;
        auto c = new MyClass;
        td[TD(c)] = 6;
        assert(TD(new MyClass) !in td);
        assert(td[TD(c)] == 6);
    }
}

@system unittest
{
    alias String = Typedef!(char[]);
    alias CString = Typedef!(const(char)[]);
    CString cs = "fubar";
    String s = cast(String) cs;
    assert(cs == s);
    char[] s2 = cast(char[]) cs;
    const(char)[] cs2 = cast(const(char)[])s;
    assert(s2 == cs2);
}

@system unittest // toString
{
    import std.meta : AliasSeq;
    import std.conv : to;

    struct TestS {}
    class TestC {}

    static foreach (T; AliasSeq!(int, bool, float, double, real,
                                 char, dchar, wchar,
                                 TestS, TestC,
                                 int*, int[], int[2], int[int]))
    {{
        T t;

        Typedef!T td;
        Typedef!(const T) ctd;
        Typedef!(immutable T) itd;

        assert(t.to!string() == td.to!string());

        static if (!(is(T == TestS) || is(T == TestC)))
        {
            assert(t.to!string() == ctd.to!string());
            assert(t.to!string() == itd.to!string());
        }
    }}
}

@safe @nogc unittest // typedef'ed type with custom operators
{
    static struct MyInt
    {
        int value;
        int opCmp(MyInt other)
        {
            if (value < other.value)
                return -1;
            return !(value == other.value);
        }
    }

    auto m1 = Typedef!MyInt(MyInt(1));
    auto m2 = Typedef!MyInt(MyInt(2));
    assert(m1 < m2);
}

/**
Allocates a `class` object right inside the current scope,
therefore avoiding the overhead of `new`. This facility is unsafe;
it is the responsibility of the user to not escape a reference to the
object outside the scope.

The class destructor will be called when the result of `scoped()` is
itself destroyed.

Scoped class instances can be embedded in a parent `class` or `struct`,
just like a child struct instance. Scoped member variables must have
type `typeof(scoped!Class(args))`, and be initialized with a call to
scoped. See below for an example.

Note:
It's illegal to move a class instance even if you are sure there
are no pointers to it. As such, it is illegal to move a scoped object.
 */
template scoped(T)
if (is(T == class))
{
    // _d_newclass now use default GC alignment (looks like (void*).sizeof * 2 for
    // small objects). We will just use the maximum of filed alignments.
    enum alignment = __traits(classInstanceAlignment, T);
    alias aligned = _alignUp!alignment;

    static struct Scoped
    {
        // Addition of `alignment` is required as `Scoped_store` can be misaligned in memory.
        private void[aligned(__traits(classInstanceSize, T) + size_t.sizeof) + alignment] Scoped_store = void;

        @property inout(T) Scoped_payload() inout
        {
            void* alignedStore = cast(void*) aligned(cast(size_t) Scoped_store.ptr);
            // As `Scoped` can be unaligned moved in memory class instance should be moved accordingly.
            immutable size_t d = alignedStore - Scoped_store.ptr;
            size_t* currD = cast(size_t*) &Scoped_store[$ - size_t.sizeof];
            if (d != *currD)
            {
                import core.stdc.string : memmove;
                memmove(alignedStore, Scoped_store.ptr + *currD, __traits(classInstanceSize, T));
                *currD = d;
            }
            return cast(inout(T)) alignedStore;
        }
        alias Scoped_payload this;

        @disable this();
        @disable this(this);

        ~this()
        {
            // `destroy` will also write .init but we have no functions in druntime
            // for deterministic finalization and memory releasing for now.
            .destroy(Scoped_payload);
        }
    }

    /** Returns the _scoped object.
    Params: args = Arguments to pass to `T`'s constructor.
    */
    @system auto scoped(Args...)(auto ref Args args)
    {
        import core.lifetime : emplace, forward;

        Scoped result = void;
        void* alignedStore = cast(void*) aligned(cast(size_t) result.Scoped_store.ptr);
        immutable size_t d = alignedStore - result.Scoped_store.ptr;
        *cast(size_t*) &result.Scoped_store[$ - size_t.sizeof] = d;
        emplace!(Unqual!T)(result.Scoped_store[d .. $ - size_t.sizeof], forward!args);
        return result;
    }
}

///
@system unittest
{
    class A
    {
        int x;
        this()     {x = 0;}
        this(int i){x = i;}
        ~this()    {}
    }

    // Standard usage, constructing A on the stack
    auto a1 = scoped!A();
    a1.x = 42;

    // Result of `scoped` call implicitly converts to a class reference
    A aRef = a1;
    assert(aRef.x == 42);

    // Scoped destruction
    {
        auto a2 = scoped!A(1);
        assert(a2.x == 1);
        aRef = a2;
        // a2 is destroyed here, calling A's destructor
    }
    // aRef is now an invalid reference

    // Here the temporary scoped A is immediately destroyed.
    // This means the reference is then invalid.
    version (Bug)
    {
        // Wrong, should use `auto`
        A invalid = scoped!A();
    }

    // Restrictions
    version (Bug)
    {
        import std.algorithm.mutation : move;
        auto invalid = a1.move; // illegal, scoped objects can't be moved
    }
    static assert(!is(typeof({
        auto e1 = a1; // illegal, scoped objects can't be copied
        assert([a1][0].x == 42); // ditto
    })));
    static assert(!is(typeof({
        alias ScopedObject = typeof(a1);
        auto e2 = ScopedObject();  // illegal, must be built via scoped!A
        auto e3 = ScopedObject(1); // ditto
    })));

    // Use with alias
    alias makeScopedA = scoped!A;
    auto a3 = makeScopedA();
    auto a4 = makeScopedA(1);

    // Use as member variable
    struct B
    {
        typeof(scoped!A()) a; // note the trailing parentheses

        this(int i)
        {
            // construct member
            a = scoped!A(i);
        }
    }

    // Stack-allocate
    auto b1 = B(5);
    aRef = b1.a;
    assert(aRef.x == 5);
    destroy(b1); // calls A's destructor for b1.a
    // aRef is now an invalid reference

    // Heap-allocate
    auto b2 = new B(6);
    assert(b2.a.x == 6);
    destroy(*b2); // calls A's destructor for b2.a
}

private size_t _alignUp(size_t alignment)(size_t n)
if (alignment > 0 && !((alignment - 1) & alignment))
{
    enum badEnd = alignment - 1; // 0b11, 0b111, ...
    return (n + badEnd) & ~badEnd;
}

// https://issues.dlang.org/show_bug.cgi?id=6580 testcase
@system unittest
{
    enum alignment = (void*).alignof;

    static class C0 { }
    static class C1 { byte b; }
    static class C2 { byte[2] b; }
    static class C3 { byte[3] b; }
    static class C7 { byte[7] b; }
    static assert(scoped!C0().sizeof % alignment == 0);
    static assert(scoped!C1().sizeof % alignment == 0);
    static assert(scoped!C2().sizeof % alignment == 0);
    static assert(scoped!C3().sizeof % alignment == 0);
    static assert(scoped!C7().sizeof % alignment == 0);

    enum longAlignment = long.alignof;
    static class C1long
    {
        long long_; byte byte_ = 4;
        this() { }
        this(long _long, ref int i) { long_ = _long; ++i; }
    }
    static class C2long { byte[2] byte_ = [5, 6]; long long_ = 7; }
    static assert(scoped!C1long().sizeof % longAlignment == 0);
    static assert(scoped!C2long().sizeof % longAlignment == 0);

    void alignmentTest()
    {
        int var = 5;
        auto c1long = scoped!C1long(3, var);
        assert(var == 6);
        auto c2long = scoped!C2long();
        assert(cast(uint)&c1long.long_ % longAlignment == 0);
        assert(cast(uint)&c2long.long_ % longAlignment == 0);
        assert(c1long.long_ == 3 && c1long.byte_ == 4);
        assert(c2long.byte_ == [5, 6] && c2long.long_ == 7);
    }

    alignmentTest();

    version (DigitalMars)
    {
        void test(size_t size)
        {
            import core.stdc.stdlib : alloca;
            cast(void) alloca(size);
            alignmentTest();
        }
        foreach (i; 0 .. 10)
            test(i);
    }
    else
    {
        void test(size_t size)()
        {
            byte[size] arr;
            alignmentTest();
        }
        static foreach (i; 0 .. 11)
            test!i();
    }
}

// Original https://issues.dlang.org/show_bug.cgi?id=6580 testcase
@system unittest
{
    class C { int i; byte b; }

    auto sa = [scoped!C(), scoped!C()];
    assert(cast(uint)&sa[0].i % int.alignof == 0);
    assert(cast(uint)&sa[1].i % int.alignof == 0); // fails
}

@system unittest
{
    class A { int x = 1; }
    auto a1 = scoped!A();
    assert(a1.x == 1);
    auto a2 = scoped!A();
    a1.x = 42;
    a2.x = 53;
    assert(a1.x == 42);
}

@system unittest
{
    class A { int x = 1; this() { x = 2; } }
    auto a1 = scoped!A();
    assert(a1.x == 2);
    auto a2 = scoped!A();
    a1.x = 42;
    a2.x = 53;
    assert(a1.x == 42);
}

@system unittest
{
    class A { int x = 1; this(int y) { x = y; } ~this() {} }
    auto a1 = scoped!A(5);
    assert(a1.x == 5);
    auto a2 = scoped!A(42);
    a1.x = 42;
    a2.x = 53;
    assert(a1.x == 42);
}

@system unittest
{
    class A { static bool dead; ~this() { dead = true; } }
    class B : A { static bool dead; ~this() { dead = true; } }
    {
        auto b = scoped!B();
    }
    assert(B.dead, "asdasd");
    assert(A.dead, "asdasd");
}

// https://issues.dlang.org/show_bug.cgi?id=8039 testcase
@system unittest
{
    static int dels;
    static struct S { ~this(){ ++dels; } }

    static class A { S s; }
    dels = 0; { scoped!A(); }
    assert(dels == 1);

    static class B { S[2] s; }
    dels = 0; { scoped!B(); }
    assert(dels == 2);

    static struct S2 { S[3] s; }
    static class C { S2[2] s; }
    dels = 0; { scoped!C(); }
    assert(dels == 6);

    static class D: A { S2[2] s; }
    dels = 0; { scoped!D(); }
    assert(dels == 1+6);
}

@system unittest
{
    // https://issues.dlang.org/show_bug.cgi?id=4500
    class A
    {
        this() { a = this; }
        this(int i) { a = this; }
        A a;
        bool check() { return this is a; }
    }

    auto a1 = scoped!A();
    assert(a1.check());

    auto a2 = scoped!A(1);
    assert(a2.check());

    a1.a = a1;
    assert(a1.check());
}

@system unittest
{
    static class A
    {
        static int sdtor;

        this() { ++sdtor; assert(sdtor == 1); }
        ~this() { assert(sdtor == 1); --sdtor; }
    }

    interface Bob {}

    static class ABob : A, Bob
    {
        this() { ++sdtor; assert(sdtor == 2); }
        ~this() { assert(sdtor == 2); --sdtor; }
    }

    A.sdtor = 0;
    scope(exit) assert(A.sdtor == 0);
    auto abob = scoped!ABob();
}

@safe unittest
{
    static class A { this(int) {} }
    static assert(!__traits(compiles, scoped!A()));
}

@system unittest
{
    static class A { @property inout(int) foo() inout { return 1; } }

    auto a1 = scoped!A();
    assert(a1.foo == 1);
    static assert(is(typeof(a1.foo) == int));

    auto a2 = scoped!(const(A))();
    assert(a2.foo == 1);
    static assert(is(typeof(a2.foo) == const(int)));

    auto a3 = scoped!(immutable(A))();
    assert(a3.foo == 1);
    static assert(is(typeof(a3.foo) == immutable(int)));

    const c1 = scoped!A();
    assert(c1.foo == 1);
    static assert(is(typeof(c1.foo) == const(int)));

    const c2 = scoped!(const(A))();
    assert(c2.foo == 1);
    static assert(is(typeof(c2.foo) == const(int)));

    const c3 = scoped!(immutable(A))();
    assert(c3.foo == 1);
    static assert(is(typeof(c3.foo) == immutable(int)));
}

@system unittest
{
    class C
    {
        this(int rval) { assert(rval == 1); }
        this(ref int lval) { assert(lval == 3); ++lval; }
    }

    auto c1 = scoped!C(1);
    int lval = 3;
    auto c2 = scoped!C(lval);
    assert(lval == 4);
}

@system unittest
{
    class C
    {
        this(){}
        this(int){}
        this(int, int){}
    }
    alias makeScopedC = scoped!C;

    auto a = makeScopedC();
    auto b = makeScopedC(1);
    auto c = makeScopedC(1, 1);

    static assert(is(typeof(a) == typeof(b)));
    static assert(is(typeof(b) == typeof(c)));
}

/**
Defines a simple, self-documenting yes/no flag. This makes it easy for
APIs to define functions accepting flags without resorting to $(D
bool), which is opaque in calls, and without needing to define an
enumerated type separately. Using `Flag!"Name"` instead of $(D
bool) makes the flag's meaning visible in calls. Each yes/no flag has
its own type, which makes confusions and mix-ups impossible.

Example:

Code calling `getLine` (usually far away from its definition) can't be
understood without looking at the documentation, even by users familiar with
the API:
----
string getLine(bool keepTerminator)
{
    ...
    if (keepTerminator) ...
    ...
}
...
auto line = getLine(false);
----

Assuming the reverse meaning (i.e. "ignoreTerminator") and inserting the wrong
code compiles and runs with erroneous results.

After replacing the boolean parameter with an instantiation of `Flag`, code
calling `getLine` can be easily read and understood even by people not
fluent with the API:

----
string getLine(Flag!"keepTerminator" keepTerminator)
{
    ...
    if (keepTerminator) ...
    ...
}
...
auto line = getLine(Yes.keepTerminator);
----

The structs `Yes` and `No` are provided as shorthand for
`Flag!"Name".yes` and `Flag!"Name".no` and are preferred for brevity and
readability. These convenience structs mean it is usually unnecessary and
counterproductive to create an alias of a `Flag` as a way of avoiding typing
out the full type while specifying the affirmative or negative options.

Passing categorical data by means of unstructured `bool`
parameters is classified under "simple-data coupling" by Steve
McConnell in the $(LUCKY Code Complete) book, along with three other
kinds of coupling. The author argues citing several studies that
coupling has a negative effect on code quality. `Flag` offers a
simple structuring method for passing yes/no flags to APIs.
 */
template Flag(string name) {
    ///
    enum Flag : bool
    {
        /**
         When creating a value of type `Flag!"Name"`, use $(D
         Flag!"Name".no) for the negative option. When using a value
         of type `Flag!"Name"`, compare it against $(D
         Flag!"Name".no) or just `false` or `0`.  */
        no = false,

        /** When creating a value of type `Flag!"Name"`, use $(D
         Flag!"Name".yes) for the affirmative option. When using a
         value of type `Flag!"Name"`, compare it against $(D
         Flag!"Name".yes).
        */
        yes = true
    }
}

///
@safe unittest
{
    Flag!"abc" flag;

    assert(flag == Flag!"abc".no);
    assert(flag == No.abc);
    assert(!flag);
    if (flag) assert(0);
}

///
@safe unittest
{
    auto flag = Yes.abc;

    assert(flag);
    assert(flag == Yes.abc);
    if (!flag) assert(0);
    if (flag) {} else assert(0);
}

/**
Convenience names that allow using e.g. `Yes.encryption` instead of
`Flag!"encryption".yes` and `No.encryption` instead of $(D
Flag!"encryption".no).
*/
struct Yes
{
    ///
    template opDispatch(string name)
    {
        enum opDispatch = Flag!name.yes;
    }
}
//template yes(string name) { enum Flag!name yes = Flag!name.yes; }

/// Ditto
struct No
{
    ///
    template opDispatch(string name)
    {
        enum opDispatch = Flag!name.no;
    }
}

///
@safe unittest
{
    Flag!"abc" flag;

    assert(flag == Flag!"abc".no);
    assert(flag == No.abc);
    assert(!flag);
    if (flag) assert(0);
}

///
@safe unittest
{
    auto flag = Yes.abc;

    assert(flag);
    assert(flag == Yes.abc);
    if (!flag) assert(0);
    if (flag) {} else assert(0);
}

/**
Detect whether an enum is of integral type and has only "flag" values
(i.e. values with a bit count of exactly 1).
Additionally, a zero value is allowed for compatibility with enums including
a "None" value.
*/
template isBitFlagEnum(E)
{
    static if (is(E Base == enum) && isIntegral!Base)
    {
        enum isBitFlagEnum = (E.min >= 0) &&
        {
            static foreach (immutable flag; EnumMembers!E)
            {{
                Base value = flag;
                value &= value - 1;
                if (value != 0) return false;
            }}
            return true;
        }();
    }
    else
    {
        enum isBitFlagEnum = false;
    }
}

///
@safe pure nothrow unittest
{
    enum A
    {
        None,
        A = 1 << 0,
        B = 1 << 1,
        C = 1 << 2,
        D = 1 << 3,
    }

    static assert(isBitFlagEnum!A);
}

/// Test an enum with default (consecutive) values
@safe pure nothrow unittest
{
    enum B
    {
        A,
        B,
        C,
        D // D == 3
    }

    static assert(!isBitFlagEnum!B);
}

/// Test an enum with non-integral values
@safe pure nothrow unittest
{
    enum C: double
    {
        A = 1 << 0,
        B = 1 << 1
    }

    static assert(!isBitFlagEnum!C);
}

/**
A typesafe structure for storing combinations of enum values.

This template defines a simple struct to represent bitwise OR combinations of
enum values. It can be used if all the enum values are integral constants with
a bit count of at most 1, or if the `unsafe` parameter is explicitly set to
Yes.
This is much safer than using the enum itself to store
the OR combination, which can produce surprising effects like this:
----
enum E
{
    A = 1 << 0,
    B = 1 << 1
}
E e = E.A | E.B;
// will throw SwitchError
final switch (e)
{
    case E.A:
        return;
    case E.B:
        return;
}
----
*/
struct BitFlags(E, Flag!"unsafe" unsafe = No.unsafe)
if (unsafe || isBitFlagEnum!(E))
{
@safe @nogc pure nothrow:
private:
    enum isBaseEnumType(T) = is(E == T);
    alias Base = OriginalType!E;
    Base mValue;

public:
    this(E flag)
    {
        this = flag;
    }

    this(T...)(T flags)
    if (allSatisfy!(isBaseEnumType, T))
    {
        this = flags;
    }

    bool opCast(B: bool)() const
    {
        return mValue != 0;
    }

    Base opCast(B)() const
    if (is(Base : B))
    {
        return mValue;
    }

    auto opUnary(string op)() const
    if (op == "~")
    {
        return BitFlags(cast(E) cast(Base) ~mValue);
    }

    auto ref opAssign(T...)(T flags)
    if (allSatisfy!(isBaseEnumType, T))
    {
        mValue = 0;
        foreach (E flag; flags)
        {
            mValue |= flag;
        }
        return this;
    }

    auto ref opAssign(E flag)
    {
        mValue = flag;
        return this;
    }

    auto ref opOpAssign(string op: "|")(BitFlags flags)
    {
        mValue |= flags.mValue;
        return this;
    }

    auto ref opOpAssign(string op: "&")(BitFlags  flags)
    {
        mValue &= flags.mValue;
        return this;
    }

    auto ref opOpAssign(string op: "|")(E flag)
    {
        mValue |= flag;
        return this;
    }

    auto ref opOpAssign(string op: "&")(E flag)
    {
        mValue &= flag;
        return this;
    }

    auto opBinary(string op)(BitFlags flags) const
    if (op == "|" || op == "&")
    {
        BitFlags result = this;
        result.opOpAssign!op(flags);
        return result;
    }

    auto opBinary(string op)(E flag) const
    if (op == "|" || op == "&")
    {
        BitFlags result = this;
        result.opOpAssign!op(flag);
        return result;
    }

    auto opBinaryRight(string op)(E flag) const
    if (op == "|" || op == "&")
    {
        return opBinary!op(flag);
    }

    bool opDispatch(string name)() const
    if (__traits(hasMember, E, name))
    {
        enum e = __traits(getMember, E, name);
        return (mValue & e) == e;
    }

    void opDispatch(string name)(bool set)
    if (__traits(hasMember, E, name))
    {
        enum e = __traits(getMember, E, name);
        if (set)
            mValue |= e;
        else
            mValue &= ~e;
    }
}

/// Set values with the | operator and test with &
@safe @nogc pure nothrow unittest
{
    enum Enum
    {
        A = 1 << 0,
    }

    // A default constructed BitFlags has no value set
    immutable BitFlags!Enum flags_empty;
    assert(!flags_empty.A);

    // Value can be set with the | operator
    immutable flags_A = flags_empty | Enum.A;

    // and tested using property access
    assert(flags_A.A);

    // or the & operator
    assert(flags_A & Enum.A);
    // which commutes.
    assert(Enum.A & flags_A);
}

/// A default constructed BitFlags has no value set
@safe @nogc pure nothrow unittest
{
    enum Enum
    {
        None,
        A = 1 << 0,
        B = 1 << 1,
        C = 1 << 2
    }

    immutable BitFlags!Enum flags_empty;
    assert(!(flags_empty & (Enum.A | Enum.B | Enum.C)));
    assert(!(flags_empty & Enum.A) && !(flags_empty & Enum.B) && !(flags_empty & Enum.C));
}

// BitFlags can be variadically initialized
@safe @nogc pure nothrow unittest
{
    import std.traits : EnumMembers;

    enum Enum
    {
        A = 1 << 0,
        B = 1 << 1,
        C = 1 << 2
    }

    // Values can also be set using property access
    BitFlags!Enum flags;
    flags.A = true;
    assert(flags & Enum.A);
    flags.A = false;
    assert(!(flags & Enum.A));

    // BitFlags can be variadically initialized
    immutable BitFlags!Enum flags_AB = BitFlags!Enum(Enum.A, Enum.B);
    assert(flags_AB.A && flags_AB.B && !flags_AB.C);

    // You can use the EnumMembers template to set all flags
    immutable BitFlags!Enum flags_all = EnumMembers!Enum;
    assert(flags_all.A && flags_all.B && flags_all.C);
}

/// Binary operations: subtracting and intersecting flags
@safe @nogc pure nothrow unittest
{
    enum Enum
    {
        A = 1 << 0,
        B = 1 << 1,
        C = 1 << 2,
    }
    immutable BitFlags!Enum flags_AB = BitFlags!Enum(Enum.A, Enum.B);
    immutable BitFlags!Enum flags_BC = BitFlags!Enum(Enum.B, Enum.C);

    // Use the ~ operator for subtracting flags
    immutable BitFlags!Enum flags_B = flags_AB & ~BitFlags!Enum(Enum.A);
    assert(!flags_B.A && flags_B.B && !flags_B.C);

    // use & between BitFlags for intersection
    assert(flags_B == (flags_BC & flags_AB));
}

/// All the binary operators work in their assignment version
@safe @nogc pure nothrow unittest
{
    enum Enum
    {
        A = 1 << 0,
        B = 1 << 1,
    }

    BitFlags!Enum flags_empty, temp, flags_AB;
    flags_AB = Enum.A | Enum.B;

    temp |= flags_AB;
    assert(temp == (flags_empty | flags_AB));

    temp = flags_empty;
    temp |= Enum.B;
    assert(temp == (flags_empty | Enum.B));

    temp = flags_empty;
    temp &= flags_AB;
    assert(temp == (flags_empty & flags_AB));

    temp = flags_empty;
    temp &= Enum.A;
    assert(temp == (flags_empty & Enum.A));
}

/// Conversion to bool and int
@safe @nogc pure nothrow unittest
{
    enum Enum
    {
        A = 1 << 0,
        B = 1 << 1,
    }

    BitFlags!Enum flags;

    // BitFlags with no value set evaluate to false
    assert(!flags);

    // BitFlags with at least one value set evaluate to true
    flags |= Enum.A;
    assert(flags);

    // This can be useful to check intersection between BitFlags
    BitFlags!Enum flags_AB = Enum.A | Enum.B;
    assert(flags & flags_AB);
    assert(flags & Enum.A);

    // You can of course get you raw value out of flags
    auto value = cast(int) flags;
    assert(value == Enum.A);
}

/// You need to specify the `unsafe` parameter for enums with custom values
@safe @nogc pure nothrow unittest
{
    enum UnsafeEnum
    {
        A = 1,
        B = 2,
        C = 4,
        BC = B|C
    }
    static assert(!__traits(compiles, { BitFlags!UnsafeEnum flags; }));
    BitFlags!(UnsafeEnum, Yes.unsafe) flags;

    // property access tests for exact match of unsafe enums
    flags.B = true;
    assert(!flags.BC); // only B
    flags.C = true;
    assert(flags.BC); // both B and C
    flags.B = false;
    assert(!flags.BC); // only C

    // property access sets all bits of unsafe enum group
    flags = flags.init;
    flags.BC = true;
    assert(!flags.A && flags.B && flags.C);
    flags.A = true;
    flags.BC = false;
    assert(flags.A && !flags.B && !flags.C);
}

// Negation of BitFlags should work with any base type.
// Double-negation of BitFlags should work.
@safe @nogc pure nothrow unittest
{
    static foreach (alias Base; AliasSeq!(
        byte,
        ubyte,
        short,
        ushort,
        int,
        uint,
        long,
        ulong,
    ))
    {{
        enum Enum : Base
        {
            A = 1 << 0,
            B = 1 << 1,
            C = 1 << 2,
        }

        auto flags = BitFlags!Enum(Enum.A);

        assert(flags == ~~flags);
    }}
}

private enum false_(T) = false;

// ReplaceType
/**
Replaces all occurrences of `From` into `To`, in one or more types `T`. For
example, `ReplaceType!(int, uint, Tuple!(int, float)[string])` yields
`Tuple!(uint, float)[string]`. The types in which replacement is performed
may be arbitrarily complex, including qualifiers, built-in type constructors
(pointers, arrays, associative arrays, functions, and delegates), and template
instantiations; replacement proceeds transitively through the type definition.
However, member types in `struct`s or `class`es are not replaced because there
are no ways to express the types resulting after replacement.

This is an advanced type manipulation necessary e.g. for replacing the
placeholder type `This` in $(REF Algebraic, std,variant).

Returns: `ReplaceType` aliases itself to the type(s) that result after
replacement.
*/
alias ReplaceType(From, To, T...) = ReplaceTypeUnless!(false_, From, To, T);

///
@safe unittest
{
    static assert(
        is(ReplaceType!(int, string, int[]) == string[]) &&
        is(ReplaceType!(int, string, int[int]) == string[string]) &&
        is(ReplaceType!(int, string, const(int)[]) == const(string)[]) &&
        is(ReplaceType!(int, string, Tuple!(int[], float))
            == Tuple!(string[], float))
    );
}

/**
Like $(LREF ReplaceType), but does not perform replacement in types for which
`pred` evaluates to `true`.
*/
template ReplaceTypeUnless(alias pred, From, To, T...)
{
    import std.meta;

    static if (T.length == 1)
    {
        static if (pred!(T[0]))
            alias ReplaceTypeUnless = T[0];
        else static if (is(T[0] == From))
            alias ReplaceTypeUnless = To;
        else static if (is(T[0] == const(U), U))
            alias ReplaceTypeUnless = const(ReplaceTypeUnless!(pred, From, To, U));
        else static if (is(T[0] == immutable(U), U))
            alias ReplaceTypeUnless = immutable(ReplaceTypeUnless!(pred, From, To, U));
        else static if (is(T[0] == shared(U), U))
            alias ReplaceTypeUnless = shared(ReplaceTypeUnless!(pred, From, To, U));
        else static if (is(T[0] == U*, U))
        {
            static if (is(U == function))
                alias ReplaceTypeUnless = replaceTypeInFunctionTypeUnless!(pred, From, To, T[0]);
            else
                alias ReplaceTypeUnless = ReplaceTypeUnless!(pred, From, To, U)*;
        }
        else static if (is(T[0] == delegate))
        {
            alias ReplaceTypeUnless = replaceTypeInFunctionTypeUnless!(pred, From, To, T[0]);
        }
        else static if (is(T[0] == function))
        {
            static assert(0, "Function types not supported," ~
                " use a function pointer type instead of " ~ T[0].stringof);
        }
        else static if (is(T[0] == U!V, alias U, V...))
        {
            template replaceTemplateArgs(T...)
            {
                static if (is(typeof(T[0]))) {   // template argument is value or symbol
                    static if (__traits(compiles, { alias _ = T[0]; }))
                        // it's a symbol
                        alias replaceTemplateArgs = T[0];
                    else
                        // it's a value
                        enum replaceTemplateArgs = T[0];
                } else
                    alias replaceTemplateArgs = ReplaceTypeUnless!(pred, From, To, T[0]);
            }
            alias ReplaceTypeUnless = U!(staticMap!(replaceTemplateArgs, V));
        }
        else static if (is(T[0] == struct))
            // don't match with alias this struct below
            // https://issues.dlang.org/show_bug.cgi?id=15168
            alias ReplaceTypeUnless = T[0];
        else static if (is(T[0] == U[], U))
            alias ReplaceTypeUnless = ReplaceTypeUnless!(pred, From, To, U)[];
        else static if (is(T[0] == U[n], U, size_t n))
            alias ReplaceTypeUnless = ReplaceTypeUnless!(pred, From, To, U)[n];
        else static if (is(T[0] == U[V], U, V))
            alias ReplaceTypeUnless =
                ReplaceTypeUnless!(pred, From, To, U)[ReplaceTypeUnless!(pred, From, To, V)];
        else
            alias ReplaceTypeUnless = T[0];
    }
    else static if (T.length > 1)
    {
        alias ReplaceTypeUnless = AliasSeq!(ReplaceTypeUnless!(pred, From, To, T[0]),
            ReplaceTypeUnless!(pred, From, To, T[1 .. $]));
    }
    else
    {
        alias ReplaceTypeUnless = AliasSeq!();
    }
}

///
@safe unittest
{
    import std.traits : isArray;

    static assert(
        is(ReplaceTypeUnless!(isArray, int, string, int*) == string*) &&
        is(ReplaceTypeUnless!(isArray, int, string, int[]) == int[]) &&
        is(ReplaceTypeUnless!(isArray, int, string, Tuple!(int, int[]))
            == Tuple!(string, int[]))
   );
}

private template replaceTypeInFunctionTypeUnless(alias pred, From, To, fun)
{
    alias RX = ReplaceTypeUnless!(pred, From, To, ReturnType!fun);
    alias PX = AliasSeq!(ReplaceTypeUnless!(pred, From, To, Parameters!fun));
    // Wrapping with AliasSeq is neccesary because ReplaceType doesn't return
    // tuple if Parameters!fun.length == 1

    string gen()
    {
        enum  linkage = functionLinkage!fun;
        alias attributes = functionAttributes!fun;
        enum  variadicStyle = variadicFunctionStyle!fun;
        alias storageClasses = ParameterStorageClassTuple!fun;

        string result;

        result ~= "extern(" ~ linkage ~ ") ";
        static if (attributes & FunctionAttribute.ref_)
        {
            result ~= "ref ";
        }

        result ~= "RX";
        static if (is(fun == delegate))
            result ~= " delegate";
        else
            result ~= " function";

        result ~= "(";
        static foreach (i; 0 .. PX.length)
        {
            if (i)
                result ~= ", ";
            if (storageClasses[i] & ParameterStorageClass.scope_)
                result ~= "scope ";
            if (storageClasses[i] & ParameterStorageClass.in_)
                result ~= "in ";
            if (storageClasses[i] & ParameterStorageClass.out_)
                result ~= "out ";
            if (storageClasses[i] & ParameterStorageClass.ref_)
                result ~= "ref ";
            if (storageClasses[i] & ParameterStorageClass.lazy_)
                result ~= "lazy ";
            if (storageClasses[i] & ParameterStorageClass.return_)
                result ~= "return ";

            result ~= "PX[" ~ i.stringof ~ "]";
        }
        static if (variadicStyle == Variadic.typesafe)
            result ~= " ...";
        else static if (variadicStyle != Variadic.no)
            result ~= ", ...";
        result ~= ")";

        static if (attributes & FunctionAttribute.pure_)
            result ~= " pure";
        static if (attributes & FunctionAttribute.nothrow_)
            result ~= " nothrow";
        static if (attributes & FunctionAttribute.property)
            result ~= " @property";
        static if (attributes & FunctionAttribute.trusted)
            result ~= " @trusted";
        static if (attributes & FunctionAttribute.safe)
            result ~= " @safe";
        static if (attributes & FunctionAttribute.nogc)
            result ~= " @nogc";
        static if (attributes & FunctionAttribute.system)
            result ~= " @system";
        static if (attributes & FunctionAttribute.const_)
            result ~= " const";
        static if (attributes & FunctionAttribute.immutable_)
            result ~= " immutable";
        static if (attributes & FunctionAttribute.inout_)
            result ~= " inout";
        static if (attributes & FunctionAttribute.shared_)
            result ~= " shared";
        static if (attributes & FunctionAttribute.return_)
            result ~= " return";
        static if (attributes & FunctionAttribute.live)
            result ~= " @live";

        return result;
    }

    mixin("alias replaceTypeInFunctionTypeUnless = " ~ gen() ~ ";");
}

@safe unittest
{
    template Test(Ts...)
    {
        static if (Ts.length)
        {
            //pragma(msg, "Testing: ReplaceType!("~Ts[0].stringof~", "
            //    ~Ts[1].stringof~", "~Ts[2].stringof~")");
            static assert(is(ReplaceType!(Ts[0], Ts[1], Ts[2]) == Ts[3]),
                "ReplaceType!("~Ts[0].stringof~", "~Ts[1].stringof~", "
                    ~Ts[2].stringof~") == "
                    ~ReplaceType!(Ts[0], Ts[1], Ts[2]).stringof);
            alias Test = Test!(Ts[4 .. $]);
        }
        else alias Test = void;
    }

    //import core.stdc.stdio;
    alias RefFun1 = ref int function(float, long);
    alias RefFun2 = ref float function(float, long);
    extern(C) int printf(const char*, ...) nothrow @nogc @system;
    extern(C) float floatPrintf(const char*, ...) nothrow @nogc @system;
    int func(float);

    int x;
    struct S1 { void foo() { x = 1; } }
    struct S2 { void bar() { x = 2; } }

    alias Pass = Test!(
        int, float, typeof(&func), float delegate(float),
        int, float, typeof(&printf), typeof(&floatPrintf),
        int, float, int function(out long, ...),
            float function(out long, ...),
        int, float, int function(ref float, long),
            float function(ref float, long),
        int, float, int function(ref int, long),
            float function(ref float, long),
        int, float, int function(out int, long),
            float function(out float, long),
        int, float, int function(lazy int, long),
            float function(lazy float, long),
        int, float, int function(out long, ref const int),
            float function(out long, ref const float),
        int, float, int function(in long, ref const int),
            float function(in long, ref const float),
        int, float, int function(long, in int),
            float function(long, in float),
        int, int, int, int,
        int, float, int, float,
        int, float, const int, const float,
        int, float, immutable int, immutable float,
        int, float, shared int, shared float,
        int, float, int*, float*,
        int, float, const(int)*, const(float)*,
        int, float, const(int*), const(float*),
        const(int)*, float, const(int*), const(float),
        int*, float, const(int)*, const(int)*,
        int, float, int[], float[],
        int, float, int[42], float[42],
        int, float, const(int)[42], const(float)[42],
        int, float, const(int[42]), const(float[42]),
        int, float, int[int], float[float],
        int, float, int[double], float[double],
        int, float, double[int], double[float],
        int, float, int function(float, long), float function(float, long),
        int, float, int function(float), float function(float),
        int, float, int function(float, int), float function(float, float),
        int, float, int delegate(float, long), float delegate(float, long),
        int, float, int delegate(float), float delegate(float),
        int, float, int delegate(float, int), float delegate(float, float),
        int, float, Unique!int, Unique!float,
        int, float, Tuple!(float, int), Tuple!(float, float),
        int, float, RefFun1, RefFun2,
        S1, S2,
            S1[1][][S1]* function(),
            S2[1][][S2]* function(),
        int, string,
               int[3] function(   int[] arr,    int[2] ...) pure @trusted,
            string[3] function(string[] arr, string[2] ...) pure @trusted,
    );

    // https://issues.dlang.org/show_bug.cgi?id=15168
    static struct T1 { string s; alias s this; }
    static struct T2 { char[10] s; alias s this; }
    static struct T3 { string[string] s; alias s this; }
    alias Pass2 = Test!(
        ubyte, ubyte, T1, T1,
        ubyte, ubyte, T2, T2,
        ubyte, ubyte, T3, T3,
    );
}

// https://issues.dlang.org/show_bug.cgi?id=17116
@safe unittest
{
    alias ConstDg = void delegate(float) const;
    alias B = void delegate(int) const;
    alias A = ReplaceType!(float, int, ConstDg);
    static assert(is(B == A));
}

 // https://issues.dlang.org/show_bug.cgi?id=19696
@safe unittest
{
    static struct T(U) {}
    static struct S { T!int t; alias t this; }
    static assert(is(ReplaceType!(float, float, S) == S));
}

 // https://issues.dlang.org/show_bug.cgi?id=19697
@safe unittest
{
    class D(T) {}
    class C : D!C {}
    static assert(is(ReplaceType!(float, float, C)));
}

// https://issues.dlang.org/show_bug.cgi?id=16132
@safe unittest
{
    interface I(T) {}
    class C : I!int {}
    static assert(is(ReplaceType!(int, string, C) == C));
}

// https://issues.dlang.org/show_bug.cgi?id=22325
@safe unittest
{
    static struct Foo(alias f) {}
    static void bar() {}
    alias _ = ReplaceType!(int, int, Foo!bar);
}

/**
Ternary type with three truth values:

$(UL
    $(LI `Ternary.yes` for `true`)
    $(LI `Ternary.no` for `false`)
    $(LI `Ternary.unknown` as an unknown state)
)

Also known as trinary, trivalent, or trilean.

See_Also:
    $(HTTP en.wikipedia.org/wiki/Three-valued_logic,
        Three Valued Logic on Wikipedia)
*/
struct Ternary
{
    @safe @nogc nothrow pure:

    private ubyte value = 6;
    private static Ternary make(ubyte b)
    {
        Ternary r = void;
        r.value = b;
        return r;
    }

    /**
        The possible states of the `Ternary`
    */
    enum no = make(0);
    /// ditto
    enum yes = make(2);
    /// ditto
    enum unknown = make(6);

    /**
     Construct and assign from a `bool`, receiving `no` for `false` and `yes`
     for `true`.
    */
    this(bool b) { value = b << 1; }

    /// ditto
    void opAssign(bool b) { value = b << 1; }

    /**
    Construct a ternary value from another ternary value
    */
    this(const Ternary b) { value = b.value; }

    /**
    $(TABLE Truth table for logical operations,
      $(TR $(TH `a`) $(TH `b`) $(TH `$(TILDE)a`) $(TH `a | b`) $(TH `a & b`) $(TH `a ^ b`))
      $(TR $(TD `no`) $(TD `no`) $(TD `yes`) $(TD `no`) $(TD `no`) $(TD `no`))
      $(TR $(TD `no`) $(TD `yes`) $(TD) $(TD `yes`) $(TD `no`) $(TD `yes`))
      $(TR $(TD `no`) $(TD `unknown`) $(TD) $(TD `unknown`) $(TD `no`) $(TD `unknown`))
      $(TR $(TD `yes`) $(TD `no`) $(TD `no`) $(TD `yes`) $(TD `no`) $(TD `yes`))
      $(TR $(TD `yes`) $(TD `yes`) $(TD) $(TD `yes`) $(TD `yes`) $(TD `no`))
      $(TR $(TD `yes`) $(TD `unknown`) $(TD) $(TD `yes`) $(TD `unknown`) $(TD `unknown`))
      $(TR $(TD `unknown`) $(TD `no`) $(TD `unknown`) $(TD `unknown`) $(TD `no`) $(TD `unknown`))
      $(TR $(TD `unknown`) $(TD `yes`) $(TD) $(TD `yes`) $(TD `unknown`) $(TD `unknown`))
      $(TR $(TD `unknown`) $(TD `unknown`) $(TD) $(TD `unknown`) $(TD `unknown`) $(TD `unknown`))
    )
    */
    Ternary opUnary(string s)()
    if (s == "~")
    {
        return make((386 >> value) & 6);
    }

    /// ditto
    Ternary opBinary(string s)(Ternary rhs)
    if (s == "|")
    {
        return make((25_512 >> (value + rhs.value)) & 6);
    }

    /// ditto
    Ternary opBinary(string s)(Ternary rhs)
    if (s == "&")
    {
        return make((26_144 >> (value + rhs.value)) & 6);
    }

    /// ditto
    Ternary opBinary(string s)(Ternary rhs)
    if (s == "^")
    {
        return make((26_504 >> (value + rhs.value)) & 6);
    }

    /// ditto
    Ternary opBinary(string s)(bool rhs)
    if (s == "|" || s == "&" || s == "^")
    {
        return this.opBinary!s(Ternary(rhs));
    }
}

///
@safe @nogc nothrow pure
unittest
{
    Ternary a;
    assert(a == Ternary.unknown);

    assert(~Ternary.yes == Ternary.no);
    assert(~Ternary.no == Ternary.yes);
    assert(~Ternary.unknown == Ternary.unknown);
}

@safe @nogc nothrow pure
unittest
{
    alias f = Ternary.no, t = Ternary.yes, u = Ternary.unknown;
    Ternary[27] truthTableAnd =
    [
        t, t, t,
        t, u, u,
        t, f, f,
        u, t, u,
        u, u, u,
        u, f, f,
        f, t, f,
        f, u, f,
        f, f, f,
    ];

    Ternary[27] truthTableOr =
    [
        t, t, t,
        t, u, t,
        t, f, t,
        u, t, t,
        u, u, u,
        u, f, u,
        f, t, t,
        f, u, u,
        f, f, f,
    ];

    Ternary[27] truthTableXor =
    [
        t, t, f,
        t, u, u,
        t, f, t,
        u, t, u,
        u, u, u,
        u, f, u,
        f, t, t,
        f, u, u,
        f, f, f,
    ];

    for (auto i = 0; i != truthTableAnd.length; i += 3)
    {
        assert((truthTableAnd[i] & truthTableAnd[i + 1])
            == truthTableAnd[i + 2]);
        assert((truthTableOr[i] | truthTableOr[i + 1])
            == truthTableOr[i + 2]);
        assert((truthTableXor[i] ^ truthTableXor[i + 1])
            == truthTableXor[i + 2]);
    }

    Ternary a;
    assert(a == Ternary.unknown);
    static assert(!is(typeof({ if (a) {} })));
    assert(!is(typeof({ auto b = Ternary(3); })));
    a = true;
    assert(a == Ternary.yes);
    a = false;
    assert(a == Ternary.no);
    a = Ternary.unknown;
    assert(a == Ternary.unknown);
    Ternary b;
    b = a;
    assert(b == a);
    assert(~Ternary.yes == Ternary.no);
    assert(~Ternary.no == Ternary.yes);
    assert(~Ternary.unknown == Ternary.unknown);
}

@safe @nogc nothrow pure
unittest
{
    Ternary a = Ternary(true);
    assert(a == Ternary.yes);
    assert((a & false) == Ternary.no);
    assert((a | false) == Ternary.yes);
    assert((a ^ true) == Ternary.no);
    assert((a ^ false) == Ternary.yes);
}

// https://issues.dlang.org/show_bug.cgi?id=22511
@safe unittest
{
    static struct S
    {
        int b;
        @disable this(this);
        this(ref return scope inout S rhs) inout
        {
            this.b = rhs.b + 1;
        }
    }

    Nullable!S s1 = S(1);
    assert(s1.get().b == 2);
    Nullable!S s2 = s1;
    assert(s2.get().b == 3);
}

@safe unittest
{
    static struct S
    {
        int b;
        this(this) { ++b; }
    }

    Nullable!S s1 = S(1);
    assert(s1.get().b == 2);
    Nullable!S s2 = s1;
    assert(s2.get().b == 3);
}

// https://issues.dlang.org/show_bug.cgi?id=24318
@system unittest
{
    static struct S
    {
        @disable this(this);
        int i;
    }

    Nullable!S s = S(1);
    assert(s.get().i == 1);
    s = S(2);
    assert(s.get().i == 2);
}

/// The old version of $(LREF SafeRefCounted), before $(LREF borrow) existed.
/// Old code may be relying on `@safe`ty of some of the member functions which
/// cannot be safe in the new scheme, and
/// can avoid breakage by continuing to use this. `SafeRefCounted` should be
/// preferred, as this type is outdated and unrecommended for new code.
struct RefCounted(T, RefCountedAutoInitialize autoInit =
    RefCountedAutoInitialize.yes)
{
    version (D_BetterC)
    {
        private enum enableGCScan = false;
    }
    else
    {
        private enum enableGCScan = hasIndirections!T;
    }

    extern(C) private pure nothrow @nogc static
    {
        pragma(mangle, "free") void pureFree( void *ptr );
        static if (enableGCScan)
            import core.memory : GC;
    }

    struct RefCountedStore
    {
        private struct Impl
        {
            T _payload;
            size_t _count;
        }

        private Impl* _store;

        private void initialize(A...)(auto ref A args)
        {
            import core.lifetime : emplace, forward;

            allocateStore();
            version (D_Exceptions) scope(failure) deallocateStore();
            emplace(&_store._payload, forward!args);
            _store._count = 1;
        }

        private void move(ref T source) nothrow pure
        {
            import std.algorithm.mutation : moveEmplace;

            allocateStore();
            moveEmplace(source, _store._payload);
            _store._count = 1;
        }

        // 'nothrow': can only generate an Error
        private void allocateStore() nothrow pure
        {
            static if (enableGCScan)
            {
                import std.internal.memory : enforceCalloc;
                _store = cast(Impl*) enforceCalloc(1, Impl.sizeof);
                GC.addRange(&_store._payload, T.sizeof);
            }
            else
            {
                import std.internal.memory : enforceMalloc;
                _store = cast(Impl*) enforceMalloc(Impl.sizeof);
            }
        }

        private void deallocateStore() nothrow pure
        {
            static if (enableGCScan)
            {
                GC.removeRange(&this._store._payload);
            }
            pureFree(_store);
            _store = null;
        }

        @property nothrow @safe pure @nogc
        bool isInitialized() const
        {
            return _store !is null;
        }

        @property nothrow @safe pure @nogc
        size_t refCount() const
        {
            return isInitialized ? _store._count : 0;
        }

        void ensureInitialized()()
        {
            // By checking for `@disable this()` and failing early we can
            // produce a clearer error message.
            static assert(__traits(compiles, { static T t; }),
                "Cannot automatically initialize `" ~ fullyQualifiedName!T ~
                "` because `" ~ fullyQualifiedName!T ~
                ".this()` is annotated with `@disable`.");
            if (!isInitialized) initialize();
        }

    }
    RefCountedStore _refCounted;

    @property nothrow @safe
    ref inout(RefCountedStore) refCountedStore() inout
    {
        return _refCounted;
    }

    this(A...)(auto ref A args)
    if (A.length > 0)
    out
    {
        assert(refCountedStore.isInitialized);
    }
    do
    {
        import core.lifetime : forward;
        _refCounted.initialize(forward!args);
    }

    this(T val)
    {
        _refCounted.move(val);
    }

    this(this) @safe pure nothrow @nogc
    {
        if (!_refCounted.isInitialized) return;
        ++_refCounted._store._count;
    }

    ~this()
    {
        if (!_refCounted.isInitialized) return;
        assert(_refCounted._store._count > 0);
        if (--_refCounted._store._count)
            return;
        // Done, destroy and deallocate
        .destroy(_refCounted._store._payload);
        _refCounted.deallocateStore();
    }

    void opAssign(typeof(this) rhs)
    {
        import std.algorithm.mutation : swap;

        swap(_refCounted._store, rhs._refCounted._store);
    }

    static if (__traits(compiles, lvalueOf!T = T.init))
    {
        void opAssign(T rhs)
        {
            import std.algorithm.mutation : move;

            static if (autoInit == RefCountedAutoInitialize.yes)
            {
                _refCounted.ensureInitialized();
            }
            else
            {
                assert(_refCounted.isInitialized);
            }
            move(rhs, _refCounted._store._payload);
        }
    }

    static if (autoInit == RefCountedAutoInitialize.yes)
    {
        //Can't use inout here because of potential mutation
        @property
        ref T refCountedPayload() return
        {
            _refCounted.ensureInitialized();
            return _refCounted._store._payload;
        }
    }

    @property nothrow @safe pure @nogc
    ref inout(T) refCountedPayload() inout return
    {
        assert(_refCounted.isInitialized, "Attempted to access an uninitialized payload.");
        return _refCounted._store._payload;
    }

    alias refCountedPayload this;

    static if (is(T == struct) && !is(typeof((ref T t) => t.toString())))
    {
        string toString(this This)()
        {
            import std.conv : to;

            static if (autoInit)
                return to!string(refCountedPayload);
            else
            {
                if (!_refCounted.isInitialized)
                    return This.stringof ~ "(RefCountedStore(null))";
                else
                    return to!string(_refCounted._store._payload);
            }
        }
    }
}

///
@betterC pure @system nothrow @nogc unittest
{
    auto rc1 = RefCounted!int(5);
    assert(rc1 == 5);
    auto rc2 = rc1;
    rc2 = 42;
    assert(rc1 == 42);
}

// More unit tests below SafeRefCounted

/**
 * Like $(LREF safeRefCounted) but used to initialize $(LREF RefCounted)
 * instead. Intended for backwards compatibility, otherwise it is preferable
 *  to use `safeRefCounted`.
 */
RefCounted!(T, RefCountedAutoInitialize.no) refCounted(T)(T val)
{
    typeof(return) res;
    res._refCounted.move(val);
    return res;
}

///
@system unittest
{
    static struct File
    {
        static size_t nDestroyed;
        string name;
        @disable this(this); // not copyable
        ~this() { name = null; ++nDestroyed; }
    }

    auto file = File("name");
    assert(file.name == "name");
    static assert(!__traits(compiles, {auto file2 = file;}));
    assert(File.nDestroyed == 0);

    {
        import std.algorithm.mutation : move;
        auto rcFile = refCounted(move(file));
        assert(rcFile.name == "name");
        assert(File.nDestroyed == 1);
        assert(file.name == null);

        auto rcFile2 = rcFile;
        assert(rcFile.refCountedStore.refCount == 2);
        assert(File.nDestroyed == 1);
    }

    assert(File.nDestroyed == 2);
}

// More unit tests below safeRefCounted