[PATCH] Add constexpr to iterator adaptors, array and range access

[Jonathan Wakely <jwakely at redhat dot com>]

This patch implements http://wg21.link/p0031 which adds constexpr to
most operations on std::reverse_iterator, std::move_iterator,
std::array, as well as std::advance, std::distance, and the
range-access functions std::begin, std::rbegin etc.

Strictly speaking, those functions should only be constexpr in C++17
and not before, but this patch makes them constexpr whenever possible.
That means the changes are fully implemented for C++14 (and the
feature-test macro is defined) but only partially implemented for
C++11, because some of the functions can't be constexpr in C++11.

My thinking is that if the committee has decided that these functions
*should* be constexpr, and changed them for C++17, then it doesn't
serve users to make them non-constexpr in C++11 and C++14 just because
the standard says so.

How do other people feel about that?

The alternative would be to define a new _GLIBCXX17_CONSTEXPR macro
and use it in all these places, so they're only constexpr in C++17
(and probably for -std=gnu++14 too, but not -std=c++14).

How strict do we want to be about obeying the "implementors can't add
constexpr" rule in these cases?

Here's the patch, but I'm not committing it yet.

	* include/bits/range_access.h (begin, end, rbegin, rend, crbegin)
	(crend): Add constexpr specifier as per P0031R0.
	* include/bits/stl_iterator.h (reverse_iterator, move_iterator)
	(__make_reverse_iterator, make_reverse_iterator, make_move_iterator):
	Add _GLIBCXX_CONSTEXPR or _GLIBCXX14_CONSTEXPR as appropriate.
	* include/bits/stl_iterator_base_funcs.h (__distance, distance)
	(__advance, advance, next, prev): Likewise.
	* include/std/array (array::begin, array::end, array::rbegin)
	(array::rend, array::cbegin, array:cend, array::crbegin)
	(array::crend, array::operator[], array::at, array::front)
	(array::back, array::data): Likewise.
	* testsuite/24_iterators/headers/iterator/range_access.cc: Add
	constexpr.
	* testsuite/24_iterators/headers/iterator/synopsis.cc: Add constexpr
	to reverse_iterator operations, advance, and distance.

Attachment: patch.txt
Description: Text document