[Bug libstdc++/118022] New: : _Copy_awaiter should explicitly construct _Yielded_decvref

[hewillk at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118022

            Bug ID: 118022
           Summary: <generator>: _Copy_awaiter should explicitly construct
                    _Yielded_decvref
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

auto
        yield_value(const _Yielded_deref& __val)
          noexcept (is_nothrow_constructible_v<_Yielded_decvref,
                    const _Yielded_deref&>)
          requires (is_rvalue_reference_v<_Yielded>
                    && constructible_from<_Yielded_decvref,
                    const _Yielded_deref&>)
        { return _Copy_awaiter(__val, _M_bottom_value()); }

We should explicitly construct _Yielded_decvref, e.g. _Yielded_decvref(__val)
as constructible_from only guarantees such validity:

#include <generator>

struct O {
  O() = default;
  explicit O(const O&) = default;
};

std::generator<O&&> gen() {
  const O o;
  co_yield o;
}

https://godbolt.org/z/8W9zGhbeM