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

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 ‘void’ 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.