[Bug libstdc++/103295] constexpr std::string does not work for clang

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103295

--- Comment #17 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:2d76292bd6719d687bc77051da265df8ed7f5a61

commit r12-5413-g2d76292bd6719d687bc77051da265df8ed7f5a61
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Nov 18 10:33:14 2021 +0000

    libstdc++: Begin lifetime of chars in constexpr std::string [PR103295]

    Clang gives errors for constexpr std::string because the memory returned
    by std::allocator<T>::allocate does not contain any objects yet, and
    attempting to set them using char_traits::assign or char_traits::copy
    fails with:

    assignment to object outside its lifetime is not allowed in a constant
expression
                  *__result = *__first;
                            ^
    This adds code to std::char_traits to use std::construct_at to begin
    lifetimes when called during constant evaluation. To support
    specializations of std::basic_string that don't use std::char_traits
    there is now another layer of wrapper around the allocator_traits, so
    that the lifetime of characters is begun as soon as the memory is
    allocated. By doing it in the char traits and allocator traits, the rest
    of basic_string can ignore the problem.

    While modifying char_traits::copy and char_traits::assign to begin
    lifetimes for the constexpr cases, I also replaced their uses of
    std::copy and std::fill_n respectively. That means we don't need
    <bits/stl_algobase.h> for char_traits.

    libstdc++-v3/ChangeLog:

            PR libstdc++/103295
            * include/bits/basic_string.h (_Alloc_traits): Replace typedef
            with struct for C++20 mode.
            * include/bits/basic_string.tcc (_M_replace): Use _Alloc_traits
            for allocation.
            * include/bits/char_traits.h (__gnu_cxx::char_traits::assign):
            Use std::_Construct during constant evaluation.
            (__gnu_cxx::char_traits::assign(CharT*, const CharT*, size_t)):
            Likewise. Replace std::fill_n with memset or manual loop.
            (__gnu_cxx::char_traits::copy): Likewise, replacing std::copy
            with memcpy.
            * include/ext/vstring.h: Include <bits/stl_algobase.h> for
            std::min.
            * include/std/string_view: Likewise.
            *
testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc:
            Add constexpr test.