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

[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;
}

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

[iains at gcc dot gnu.org]

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

Iain Sandoe <iains at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |iains at gcc dot gnu.org
   Target Milestone|---                         |10.0
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |rejects-valid
   Last reconfirmed|                            |2020-04-25

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Iain D Sandoe <iains@gcc.gnu.org>:

https://gcc.gnu.org/g:b9c91b7f3279e23aed965c05197acf3b6f439f8d

commit r10-8004-gb9c91b7f3279e23aed965c05197acf3b6f439f8d
Author: Iain Sandoe <iain@sandoe.co.uk>
Date:   Tue Apr 28 00:27:47 2020 +0100

    coroutines: Fix handling of non-class coroutine returns [PR94759]

    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 âvoidâ is not a class'

    This is because the check for non-class-ness of the return value was
    in the wrong place; it needs to be carried out in a SFINAE context.

    The following patch removes the restriction in the traits template
    instantiation and allows for the case that the ramp function could
    return void.

    The <coroutine> header is amended to implement the required
    functionality.

    gcc/cp/ChangeLog:

    2020-04-28  Iain Sandoe  <iain@sandoe.co.uk>

            PR c++/94759
            * coroutines.cc (coro_promise_type_found_p): Do not
            exclude non-classes here (this needs to be handled in the
            coroutine header).
            (morph_fn_to_coro):  Allow for the case where the coroutine
            returns void.

    gcc/testsuite/ChangeLog:

    2020-04-28  Iain Sandoe  <iain@sandoe.co.uk>

            PR c++/94759
            * g++.dg/coroutines/coro-bad-alloc-00-bad-op-new.C: Adjust for
            updated error messages.
            * g++.dg/coroutines/coro-bad-alloc-01-bad-op-del.C: Likewise.
            * g++.dg/coroutines/coro-bad-alloc-02-no-op-new-nt.C: Likewise.
            * g++.dg/coroutines/coro-missing-promise.C: Likewise.
            * g++.dg/coroutines/pr93458-5-bad-coro-type.C: Liekwise.
            * g++.dg/coroutines/torture/co-ret-17-void-ret-coro.C: New test.

    libstdc++-v3/ChangeLog:

    2020-04-28  Jonathan Wakely  <jwakely@redhat.com>
                Iain Sandoe  <iain@sandoe.co.uk>

            PR c++/94759
            * include/std/coroutine: Implement handing for non-
            class coroutine return types.

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

[iains at gcc dot gnu.org]

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

Iain Sandoe <iains at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #2 from Iain Sandoe <iains at gcc dot gnu.org> ---
fixed on master.