[PATCH] Implement C++17 changes to std::atomic (P0558R1 partial)

[Jonathan Wakely <jwakely at redhat dot com>]

This implements the significant changes from P0558R1 for C++17, adding
nested typedefs and making it ill-formed to do arithmetic on
std::atomic<T*> if !is_object<T>::value.

We currently support such arithmetic for std::atomic<void*>, and even
have a test checking it, so I've only made it ill-formed for C++17
mode. I'd like to deprecate that extension for GCC 8 and make it
ill-formed in all modes at some point. PR 69769 points out that we
also allow that arithmetic for pointers to functions, which is just
crazy.

The unimplemented parts of the paper are changing the non-member
functions to use the new std::atomic<T>::value_type and
std::atomic<T>::difference_type types for the parameters, so they are
non-deduced contexts. That would be too big a change to make now, so
we can revisit that for GCC 8 too.

	PR libstdc++/69769
	* include/bits/atomic_base.h (__atomic_base): Add new typedefs for
	C++17.
	* include/std/atomic (atomic<bool>, atomic<T>, atomic<T*>): Likewise.
	(atomic<T*>::operator++, atomic<T*>::operator--)
	(atomic<T*>::operator+=, atomic<T*>::operator-=)
	(atomic<T*>::fetch_add, atomic<T*>::fetch_sub): Add static assertion
	to enforce C++17 requirement on pointer arithmetic.
	* testsuite/29_atomics/atomic/60695.cc: Adjust dg-error lineno.
	* testsuite/29_atomics/atomic/69769.cc: New test.
	* testsuite/29_atomics/atomic/operators/pointer_partial_void.cc:
	Disable test for C++17.
	* testsuite/29_atomics/atomic/requirements/typedefs.cc: New test.
	* testsuite/29_atomics/atomic_integral/requirements/typedefs.cc: New
	test.

Tested powerpc64le-linux, with all -std=gnu++?? options.

I plan to commit this tomorrow.

Attachment: patch.txt
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