[Bug libstdc++/95048] [11 Regression] wstring-constructor of std::filesystem::path throws for non-ASCII characters

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95048

--- Comment #26 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:d308b11fa94728507984b4ccc949219511273ab6

commit r11-10903-gd308b11fa94728507984b4ccc949219511273ab6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Nov 11 15:22:02 2022 +0000

    libstdc++: Fix wstring conversions in filesystem::path [PR95048]

    In commit r9-7381-g91756c4abc1757 I changed filesystem::path to use
    std::codecvt<CharT, char, mbstate_t> for conversions from all wide
    strings to UTF-8, instead of using std::codecvt_utf8<CharT>. This was
    done because for 16-bit wchar_t, std::codecvt_utf8<wchar_t> only
    supports UCS-2 and not UTF-16. The rationale for the change was sound,
    but the actual fix was not. It's OK to use std::codecvt for char16_t or
    char32_t, because the specializations for those types always use UTF-8 ,
    but std::codecvt<wchar_t, char, mbstate_t> uses the current locale's
    encodings, and the narrow encoding is probably ASCII and can't support
    non-ASCII characters.

    The correct fix is to use std::codecvt only for char16_t and char32_t.
    For 32-bit wchar_t we could have continued using std::codecvt_utf8
    because that uses UTF-32 which is fine, switching to std::codecvt broke
    non-Windows targets with 32-bit wchar_t. For 16-bit wchar_t we did need
    to change, but should have changed to std::codecvt_utf8_utf16<wchar_t>
    instead, as that always uses UTF-16 not UCS-2. I actually noted that in
    the commit message for r9-7381-g91756c4abc1757 but didn't use that
    option. Oops.

    This replaces the unconditional std::codecvt<CharT, char, mbstate_t>
    with a type defined via template specialization, so it can vary
    depending on the wide character type. The code is also simplified to
    remove some of the mess of #ifdef and if-constexpr conditions.

    libstdc++-v3/ChangeLog:

            PR libstdc++/95048
            * include/bits/fs_path.h (path::_Codecvt): New class template
            that selects the kind of code conversion done.
            (path::_Codecvt<wchar_t>): Select based on sizeof(wchar_t).
            (_GLIBCXX_CONV_FROM_UTF8): New macro to allow the same code to
            be used for Windows and POSIX.
            (path::_S_convert(const EcharT*, const EcharT*)): Simplify by
            using _Codecvt and _GLIBCXX_CONV_FROM_UTF8 abstractions.
            (path::_S_str_convert(basic_string_view<value_type>, const A&)):
            Simplify nested conditions.
            * include/experimental/bits/fs_path.h (path::_Cvt): Define
            nested typedef controlling type of code conversion done.
            (path::_Cvt::_S_wconvert): Use new typedef.
            (path::string(const A&)): Likewise.
            * testsuite/27_io/filesystem/path/construct/95048.cc: New test.
            * testsuite/experimental/filesystem/path/construct/95048.cc: New
            test.

    (cherry picked from commit b331bf303bdc1edead41e2b3d11d1a7804b433cf)