true iff the type T can be tested in an if-expression, that is if if (pred(T.init)) {} is compilable.
class C; struct S1; struct S2 { T opCast(T)() const; } static assert( ifTestable!bool); static assert( ifTestable!int); static assert( ifTestable!(S1*)); static assert( ifTestable!(typeof(null))); static assert( ifTestable!(int[])); static assert( ifTestable!(int[string])); static assert( ifTestable!S2); static assert( ifTestable!C); static assert(!ifTestable!S1);