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.
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