[Bug libstdc++/118395] Constructor of std::barrier is not constexpr

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118395

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We also have:

      static constexpr ptrdiff_t
      max() noexcept
      { return __PTRDIFF_MAX__; }


But this seems wrong, because the constructor does:

        size_t const __count = (_M_expected + 1) >> 1;

This means that std::barrier{std::barrier::max()} would overflow, with
undefined behaviour.