[Bug c++/123955] ‘Reference to void’ error for C++20 partial specialization of class template with void

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123955

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Htamak R G from comment #0)
> Workaround:
> Scoping all concepts in a requires block to delay substitution.
> 
> ```cpp
> template <typename T, typename CloseStatus>
> concept InvariantClassLike = requires {
>   requires std::movable<T> &&
>                (std::same_as<CloseStatus, void> ||
> std::copyable<CloseStatus>);
> };
> ```

You only need to do it for the one that causes a problem:

template <typename T, typename CloseStatus>
concept InvariantClassLike =
    std::movable<T> &&
    (std::same_as<CloseStatus, void>
     || requires { requires std::copyable<CloseStatus>; });