[Bug c++/94759] New: coroutines: rejects traits specialisation with non-class returns.

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94759

            Bug ID: 94759
           Summary: coroutines: rejects traits specialisation with
                    non-class returns.
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iains at gcc dot gnu.org
  Target Milestone: ---

Created attachment 48373
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48373&action=edit
fix under test

>From the standard:

The header <coroutine> defines the primary template coroutine_traits such that
if ArgTypes is a parameter pack of types and if the qualified-id
R::promise_type is valid and denotes a type, then
coroutine_traits<R,ArgTypes...> has the following publicly accessible member:
     using promise_type = typename R::promise_type;

this should not prevent more specialised cases and  the following code should
be accepted, but is currently rejected with

 error: coroutine return type ‘void’ is not a class

#include <coroutine>

template<typename R, typename HandleRef, typename ...T>
struct std::coroutine_traits<R, HandleRef, T...> {
    struct promise_type {
        promise_type (HandleRef h, T ...args)
        { h = std::coroutine_handle<promise_type>::from_promise (*this);
          PRINT ("Created Promise");
        }

        void get_return_object() {}

        auto initial_suspend() {
          return std::suspend_always{};
         }
        auto final_suspend() { return std::suspend_never{}; }

        void return_void() {}
        void unhandled_exception() {}
    };
};

void
my_coro (std::coroutine_handle<>& h)
{
  PRINT ("coro1: about to return");
  co_return;
}