Re: [committed] libstdc++: Add std::is_scoped_enum for C++23

[Tim Song <t.canens.cpp@gmail.com>]

On Fri, Mar 19, 2021 at 3:13 PM Jonathan Wakely via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Implement this C++23 feature, as proposed by P1048R1.
>
> This implementation assumes that a C++23 compiler supports concepts
> already. I don't see any point in using preprocessor hacks to detect
> compilers which define __cplusplus to a post-C++20 value but don't
> support concepts yet.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/type_traits (is_scoped_enum): Define.
>         * include/std/version (__cpp_lib_is_scoped_enum): Define.
>         * testsuite/20_util/is_scoped_enum/value.cc: New test.
>         * testsuite/20_util/is_scoped_enum/version.cc: New test.
>
> Tested powerpc64le-linux. Committed to trunk.
>

Using __underlying_type breaks for incomplete enumeration types. GCC
doesn't have incomplete scoped enums due to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89025 but unscoped ones
exist:

enum E {
    x = std::is_scoped_enum_v<E>
};