[Bug libstdc++/120259] Using Incomplete&& as argument in invoke_r/move_only_function fails to compile

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120259

--- Comment #1 from Tomasz Kamiński <tkaminsk at gcc dot gnu.org> ---
We should use _Arg&& in is_invokable checks here:
```
  template<typename _Res, typename _Callable, typename... _Args>
    constexpr enable_if_t<is_invocable_r_v<_Res, _Callable, _Args...>, _Res>
    __invoke_r(_Callable&& __fn, _Args&&... __args)
    noexcept(is_nothrow_invocable_r_v<_Res, _Callable, _Args...>);
```