[committed 2/2] libstdc++: Implement constexpr std::basic_string for C++20

[Jonathan Wakely]

Oops, the subject line was not supposed to say 2/2 for this commit, and I
was not supposed to have Michael de Lang as the author ... I messed up my
git send-email and git cherry-pick commands!

Sorry Michael, I originally tried to use your tests from
https://github.com/Oipo/gcc/ but as noted in https://gcc.gnu.org/PR93989
those tests are incorrect, and so I didn't actually use any of them (nor
the std::string code itself). But apparently the commit still had you as
the author, because I reset the content of the git tree, but not the commit
author. I'll fix that in GCC's ChangeLog file after it regenerates
overnight.



On Tue, 16 Nov 2021 at 16:47, Jonathan Wakely wrote:

> From: Michael de Lang
>
> Tested x86_64-linux, committed to trunk.
>
>
> 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.
>
>
>