[Bug c++/95346] New: [coroutines] coroutine return-type should be initialised with rvalue if different from get_return_object() return-type

["lewissbaker.opensource at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95346

            Bug ID: 95346
           Summary: [coroutines] coroutine return-type should be
                    initialised with rvalue if different from
                    get_return_object() return-type
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lewissbaker.opensource at gmail dot com
  Target Milestone: ---

If I have the following code:

----------
#include <coroutine>

struct task {
    struct promise_type {
        task get_return_object();
        void return_void();
        void unhandled_exception();
        std::suspend_always initial_suspend() noexcept;
        std::suspend_always final_suspend() noexcept;
    };
};

struct wrapper {
    using promise_type = task::promise_type;
    wrapper(task&&);
};

wrapper f() {
    co_return;
}
----------


Then building with GCC 10.0 and GCC trunk with -std=c++20 -fcoroutines fails
with the following compile-error:


x86-64 gcc (trunk)
-O3 -std=c++2a -Wall -fcoroutines
1
<Compilation failed>
x86-64 gcc (trunk) - 368ms
<source>: In function 'wrapper f()':

error: cannot bind rvalue reference of type 'task&&' to lvalue of type 'task'
   30 | }
      | ^
note:   initializing argument 1 of 'wrapper::wrapper(task&&)'
   25 |     wrapper(task&&);


I think the return-value of a coroutine should be initialised with an
rvalue-reference to the object returned from get_return_object() if the
two objects have different types. GCC seems to be trying to initialise
the return-value using an lvalue-reference to the object returned from
get_return_object().

Both Clang and MSVC accept this code, while GCC does not.
See https://godbolt.org/z/4RRtaL