static assert(!hasLength!(char[]));
static assert( hasLength!(int[]));
static assert( hasLength!(inout(int)[]));
struct A { size_t length() { return 0; } }
struct B { @property size_t length() { return 0; } }
static assert( hasLength!(A));
static assert( hasLength!(B));
Yields true if R has a length member that returns a value of size_t type. R does not have to be a range. If R is a range, algorithms in the standard library are only guaranteed to support length with type size_t.
Note that length is an optional primitive as no range must implement it. Some ranges do not store their length explicitly, some cannot compute it without actually exhausting the range (e.g. socket streams), and some other ranges may be infinite.
Although narrow string types (char[], wchar[], and their qualified derivatives) do define a length property, hasLength yields false for them. This is because a narrow string's length does not reflect the number of characters, but instead the number of encoding units, and as such is not useful with range-oriented algorithms. To use strings as random-access ranges with length, use std.string.representation or std.utf.byCodeUnit.