A call to `basic_string::clear()` in the std::string move assignment
operator leads to a constexpr error from an access of inactive union
member `_M_local_buf` in the added test (`test_move()`). Changing
`__str._M_local_buf` to `__str._M_use_local_data()` in
`operator=(basic_string&& __str)` fixes this.

	PR libstdc++/113294

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string::operator=): Use
	_M_use_local_data() instead of _M_local_buf on the moved-from
	string.
	* testsuite/21_strings/basic_string/modifiers/constexpr.cc
	(test_move): New test.

Signed-off-by: Paul Keir <paul.keir@uws.ac.uk>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

The testsuite will automatically select C++20 for these tests now, and
removing the hardcoded -std option allows them to be tested for C++23
and C++26 as well.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/basic_string/cons/char/105995.cc: Remove
	dg-options -std=gnu++20.
	* testsuite/21_strings/basic_string/cons/char/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string/erasure.cc: Likewise.
	* testsuite/21_strings/basic_string/hash/hash_char8_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string/literals/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/swap/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/ends_with/char.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/ends_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/ends_with/wchar_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/starts_with/char.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/starts_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/starts_with/wchar_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operators/char/cmp_c++20.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operators/wchar_t/cmp_c++20.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/cons/char/range.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/copy/char/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/copy/wchar_t/constexpr.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/ends_with/char.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/ends_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/ends_with/wchar_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/starts_with/char.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/starts_with/nonnull.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operations/starts_with/wchar_t.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operators/char/cmp_c++20.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/operators/wchar_t/cmp_c++20.cc:
	Likewise.
	* testsuite/21_strings/basic_string_view/requirements/constexpr_iter.cc:
	Likewise.
	* testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc:
	Likewise.

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.

Some tests fail when run with -D_GLIBCXX_USE_CXX11_ABI or -stdgnu++20.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (operator<=>): Use constexpr
	unconditionally.
	* testsuite/21_strings/basic_string/modifiers/constexpr.cc:
	Require cxx11-abit effective target.
	* testsuite/21_strings/headers/string/synopsis.cc: Add
	conditional constexpr to declarations, and adjust relational
	operators for C++20.

This is only supported for the cxx11 ABI, not for COW strings.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string, operator""s): Add
	constexpr for C++20.
	(basic_string::basic_string(basic_string&&)): Only copy
	initialized portion of the buffer.
	(basic_string::basic_string(basic_string&&, const Alloc&)):
	Likewise.
	* include/bits/basic_string.tcc (basic_string): Add constexpr
	for C++20.
	(basic_string::swap(basic_string&)): Only copy initialized
	portions of the buffers.
	(basic_string::_M_replace): Add constexpr implementation that
	doesn't depend on pointer comparisons.
	* include/bits/cow_string.h: Adjust comment.
	* include/ext/type_traits.h (__is_null_pointer): Add constexpr.
	* include/std/string (erase, erase_if): Add constexpr.
	* include/std/version (__cpp_lib_constexpr_string): Update
	value.
	* testsuite/21_strings/basic_string/cons/char/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/literals/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/modifiers/constexpr.cc: New test.
	* testsuite/21_strings/basic_string/modifiers/swap/char/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/modifiers/swap/wchar_t/constexpr.cc:
	New test.
	* testsuite/21_strings/basic_string/version.cc: New test.