chrono locale format code has some bug when using RHEL gcc on CentOS 7.6 to compile

[g199209]

Test version : 10.2.1

Test code snippet:

std::locale::global(loc);
auto sat = fmt::weekday(6);
fmt::format(loc, "{:L}", sat);

The code above will throw std::bad_cast when using specifed compiler and OS.

Compiler: g++ (GCC) 12.2.1 20221121 (Red Hat 12.2.1-4)
OS: CentOS 7.6

This gcc is from RHEL Dev tool set 12: https://access.redhat.com/documentation/en-us/red_hat_developer_toolset/12

---

Root cause: This compile has predefined #define _GLIBCXX_USE_DUAL_ABI 0, the old CentOS 7.6 do not support dual ABI, but the _GLIBCXX_USE_DUAL_ABI IS defined to 0, not undefined.

And related code in include/fmt/chrono.h:

template <typename OutputIt>
auto write_encoded_tm_str(OutputIt out, string_view in, const std::locale& loc)
    -> OutputIt {
  if (detail::is_utf8() && loc != get_classic_locale()) {
    // char16_t and char32_t codecvts are broken in MSVC (linkage errors) and
    // gcc-4.
#if FMT_MSC_VERSION != 0 || \
    (defined(__GLIBCXX__) && !defined(_GLIBCXX_USE_DUAL_ABI))
    // The _GLIBCXX_USE_DUAL_ABI macro is always defined in libstdc++ from gcc-5
    // and newer.
    using code_unit = wchar_t;
#else
    using code_unit = char32_t;
#endif

    using unit_t = codecvt_result<code_unit>;
    unit_t unit;
    write_codecvt(unit, in, loc);
    // ......
}

If _GLIBCXX_USE_DUAL_ABI IS defined, std::use_facet<std::codecvt<char32_t, char, std::mbstate_t>>(loc) will be called in write_codecvt(), which throw std::bad_cast. If I only keep using code_unit = wchar_t; here, the code above can work well.

I think we should not only check whether _GLIBCXX_USE_DUAL_ABI is defined, but also check it's value here:

#if FMT_MSC_VERSION != 0 || \
    (defined(__GLIBCXX__) && (!defined(_GLIBCXX_USE_DUAL_ABI) || _GLIBCXX_USE_DUAL_ABI == 0 ))