[Bug libstdc++/99181] char_traits<char> (and thus string_view) compares strings differently in constexpr and non-constexpr contexts

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-8 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:21a7edf3a0cea656f2de834ea8320870496d5a2f

commit r8-10889-g21a7edf3a0cea656f2de834ea8320870496d5a2f
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Feb 23 09:30:18 2021 +0100

    libstdc++: Fix up constexpr std::char_traits<char>::compare [PR99181]

    Because of LWG 467, std::char_traits<char>::lt compares the values
    cast to unsigned char rather than char, so even when char is signed
    we get unsigned comparision.  std::char_traits<char>::compare uses
    __builtin_memcmp and that works the same, but during constexpr evaluation
    we were calling __gnu_cxx::char_traits<char_type>::compare.  As
    char_traits::lt is not virtual, __gnu_cxx::char_traits<char_type>::compare
    used __gnu_cxx::char_traits<char_type>::lt rather than
    std::char_traits<char>::lt and thus compared chars as signed if char is
    signed.
    This change fixes it by inlining __gnu_cxx::char_traits<char_type>::compare
    into std::char_traits<char>::compare by hand, so that it calls the right
    lt method.

    2021-02-23  Jakub Jelinek  <jakub@redhat.com>

            PR libstdc++/99181
            * include/bits/char_traits.h (char_traits<char>::compare): For
            constexpr evaluation don't call
            __gnu_cxx::char_traits<char_type>::compare but do the comparison
loop
            directly.

            * testsuite/21_strings/char_traits/requirements/char/99181.cc: New
            test.

    (cherry picked from commit 311c57f6d8f285d69e44bf94152c753900cb1a0a)