I made a last-minute change to Nina's r10-200-gf4e678ef74b272
implementation of P1165R1 (consistent allocator propagation for
operator+ on strings), so that the rvalue+rvalue case assumes that COW
strings do not support stateful allocators. I don't think that was true
when the change went in, and isn't true now. COW strings don't support
allocator propagation on assignment and swap, but they do support
non-equal stateful allocators, which are correctly propagated on move
construction.

This removes the preprocessor conditional in the rvalue+rvalue overload
so that COW strings are handled equivalently. Also use constexpr-if
unconditionally, disabling diagnostics with pragmas.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (operator+(string&&, string&&)):
	Do not assume that COW strings have equal allocators. Use
	constexpr-if unconditionally.
	* testsuite/21_strings/basic_string/allocator/char/operator_plus.cc:
	Remove cxx11_abi effective-target check.
	* testsuite/21_strings/basic_string/allocator/wchar_t/operator_plus.cc:
	Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

The changes to implement LWG 2579 (r10-327-gdb33efde17932f) made
std::string::assign use the propagate_on_container_copy_assignment
(POCCA) trait, for consistency with operator=(const basic_string&).
However, this also unintentionally affected operator=(basic_string&&)
which calls assign(str) to make a deep copy when performing a move is
not possible. The fix is for the move assignment operator to call
_M_assign(str) instead of assign(str), as this just does the deep copy
and doesn't check the POCCA trait first.

The bug only affects the unlikely/useless combination of POCCA==true and
POCMA==false, but we should fix it for correctness anyway. it should
also make move assignment slightly cheaper to compile and execute,
because we skip the extra code in assign(const basic_string&).

libstdc++-v3/ChangeLog:

	PR libstdc++/116641
	* include/bits/basic_string.h (operator=(basic_string&&)): Call
	_M_assign instead of assign.
	* testsuite/21_strings/basic_string/allocator/116641.cc: New
	test.

Since resolution of Issue 2593 [1] we can consider that equal allocators
before the propagate-on-move-assignment operations will still be equal
afterward.

So we can extend the optimization of transfering the storage of the move-to
instance to the move-from one that is currently limited to always equal
allocators.

[1] https://cplusplus.github.io/LWG/issue2593

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (operator=(basic_string&&)): Transfer move-to
	storage to the move-from instance when allocators are equal.
	* testsuite/21_strings/basic_string/allocator/char/move_assign.cc (test04):
	New test case.

An effective target like foo-bar-baz will match a target selector of
*-*-* and cause problems in the testsuite. Several libstdc++ et keywords
are of the form foo-bar, which could still be a problem for *-*
selectors.

Replace hyphens with underscores in the et keywords "debug-mode",
"cxx11-abi", etc.

libstdc++-v3/ChangeLog:

	* testsuite/lib/libstdc++.exp: Rename effective target keywords
	to avoid dashes in the name.
	* testsuite/*: Update effective targe keywords.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/basic_string/allocator/71964.cc: Fix
	typo.
	* testsuite/23_containers/set/allocator/71964.cc: Likewise.