[Bug c++/95350] New: [coroutines] coroutines with move-only by-value parameters attempt to copy parameter instead of move it

[Bug c++/95350] New: [coroutines] coroutines with move-only by-value parameters attempt to copy parameter instead of move it

[lewissbaker.opensource at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95350

            Bug ID: 95350
           Summary: [coroutines] coroutines with move-only by-value
                    parameters attempt to copy parameter instead of move
                    it
           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: ---

The following code fails to compile under GCC 10.0 and GCC trunk.
Compile with -fcoroutines -std=c++2a

See https://godbolt.org/z/zB_RVC

This code compiles successfully under both Clang and MSVC coroutines
implementations.

-------------
#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 move_only {
    move_only();
    move_only(const move_only&) = delete;
    move_only(move_only&) = delete;
    move_only(move_only&&) = default;
};

task f(move_only x) {
    co_return;
}

---------------------

Fails with compile-error:

<source>: In function 'task f(move_only)':
<source>:30:1: error: use of deleted function 'move_only::move_only(const
move_only&)'
   30 | }
      | ^
<source>:24:5: note: declared here
   24 |     move_only(move_only&) = delete;
      |     ^~~~~~~~~

When copying parameters into the coroutine frame which are passed by value
it should be initialising the parameter copy using an xvalue referring to the
original function parameter. Whereas it seems like GCC is attempting to
initialise the parameter-copy with an lvalue-reference to the original
parameter, which ends up calling the copy-constructor.

See http://eel.is/c++draft/dcl.fct.def.coroutine#13
> ... the copy is a variable of type cv T with automatic storage duration
> that is direct-initialized from an xvalue of type T referring to the
> parameter ...

[Bug c++/95350] [coroutines] coroutines with move-only by-value parameters attempt to copy parameter instead of move it

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95350

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.2
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
           Keywords|                            |wrong-code
   Last reconfirmed|                            |2020-05-27

--- Comment #1 from Iain Sandoe <iains at gcc dot gnu.org> ---
thanks for the report, I have a fix for this in my patch queue (but the
reproducer test-case is very useful).

[Bug c++/95350] [coroutines] coroutines with move-only by-value parameters attempt to copy parameter instead of move it

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95350

--- Comment #2 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:88f48e2967ead9be262483618238efa9c7c842ec

commit r11-773-g88f48e2967ead9be262483618238efa9c7c842ec
Author: Iain Sandoe <iain@sandoe.co.uk>
Date:   Mon Jun 1 08:28:35 2020 +0100

    coroutines: Correct handling of references in parm copies [PR95350].

    Adjust to handle rvalue refs the same way as clang, and to correct
    the handling of moves when a copy CTOR is present.  This is one area
    where we could make things easier for the end-user (as was implemented
    before this change), however there needs to be agreement about when the
    full statement containing a coroutine call ends (i.e. when the ramp
    terminates or when the coroutine terminates).

    gcc/cp/ChangeLog:

            PR c++/95350
            * coroutines.cc (struct param_info): Remove rv_ref field.
            (build_actor_fn): Remove specifial rvalue ref handling.
            (morph_fn_to_coro): Likewise.

    gcc/testsuite/ChangeLog:

            PR c++/95350
            * g++.dg/coroutines/torture/func-params-08.C: Adjust test to
            reflect that all rvalue refs are dangling.
            * g++.dg/coroutines/torture/func-params-09-awaitable-parms.C:
            Likewise.
            * g++.dg/coroutines/pr95350.C: New test.

[Bug c++/95350] [coroutines] coroutines with move-only by-value parameters attempt to copy parameter instead of move it

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95350

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Iain D Sandoe
<iains@gcc.gnu.org>:

https://gcc.gnu.org/g:284f809ef7fe8fa6b518c103d31e514a98f0f36e

commit r10-8285-g284f809ef7fe8fa6b518c103d31e514a98f0f36e
Author: Iain Sandoe <iain@sandoe.co.uk>
Date:   Fri Jun 12 08:28:35 2020 +0100

    coroutines: Correct handling of references in parm copies [PR95350].

    Adjust to handle rvalue refs the same way as clang, and to correct
    the handling of moves when a copy CTOR is present.  This is one area
    where we could make things easier for the end-user (as was implemented
    before this change), however there needs to be agreement about when the
    full statement containing a coroutine call ends (i.e. when the ramp
    terminates or when the coroutine terminates).

    gcc/cp/ChangeLog:

            PR c++/95350
            * coroutines.cc (struct param_info): Remove rv_ref field.
            (build_actor_fn): Remove specifial rvalue ref handling.
            (morph_fn_to_coro): Likewise.

    gcc/testsuite/ChangeLog:

            PR c++/95350
            * g++.dg/coroutines/torture/func-params-08.C: Adjust test to
            reflect that all rvalue refs are dangling.
            * g++.dg/coroutines/torture/func-params-09-awaitable-parms.C:
            Likewise.
            * g++.dg/coroutines/pr95350.C: New test.

    (cherry picked from commit 88f48e2967ead9be262483618238efa9c7c842ec)

[Bug c++/95350] [coroutines] coroutines with move-only by-value parameters attempt to copy parameter instead of move it

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95350

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

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

--- Comment #4 from Iain Sandoe <iains at gcc dot gnu.org> ---
fixed for maater and 10.2.