[Bug c++/115908] New: [coroutines] Wrong behavior of using get_return_object() coroutines creation

[ddvamp007 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115908

            Bug ID: 115908
           Summary: [coroutines] Wrong behavior of using
                    get_return_object() coroutines creation
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ddvamp007 at gmail dot com
  Target Milestone: ---

The standard says: "The expression promise.get_return_object() is used to
initialize the returned reference or value result object of a call to a
coroutine. The call to get_return_object is sequenced before the call to
initial_suspend and is invoked at most once." [dcl.fct.def.coroutine]
However, if you look at the result of the following program
https://godbolt.org/z/nY8Yo6vGs, it can be seen that the standard is not being
followed.
Firstly, it is not the expression itself that is used for initialization, but
its result
Secondly, if this result is a reference, it is still stored as a value

As a consequence, the behavior is not what the user expects. If we return an
non-copyable and non-movable object type referece, we will get a compilation
error. If the result contains references to the promise object, it may be
destroyed before it is used. etc.

P.S. The same thing is observed in clang