[Bug c++/105406] New: coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[Bug c++/105406] New: coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[dvratil at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105406

            Bug ID: 105406
           Summary: coroutines: since 11.3 co_await attempts to copy a
                    move-only value when await_transform(T &) exists
           Product: gcc
           Version: 11.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dvratil at kde dot org
  Target Milestone: ---

Starting with gcc 11.3, gcc tries to copy an object to await_transform() even
if an overload that accepts a reference exists:

#include <coroutine>
#include <exception>

// A move-only awaitable
class MoveOnlyAwaitable {
public:
    MoveOnlyAwaitable() = default;
    MoveOnlyAwaitable(MoveOnlyAwaitable &&) = default;
    MoveOnlyAwaitable &operator=(MoveOnlyAwaitable &&) = default;

    MoveOnlyAwaitable(const MoveOnlyAwaitable &) = delete;
    MoveOnlyAwaitable &operator=(const MoveOnlyAwaitable &) = delete;

    bool await_ready() const noexcept { return false; }
    void await_suspend(std::coroutine_handle<>) noexcept {}
    void await_resume() {}
};

struct task {
    struct promise_type {
        auto initial_suspend() const { return std::suspend_never{}; }
        auto final_suspend() const noexcept { return std::suspend_never(); }
        auto get_return_object() { return task{}; }
        void return_void() {}
        void unhandled_exception() {}

        template<typename T>
        T &&await_transform(T &&t) {
            return static_cast<T &&>(t);
        }


    };

    bool await_ready() const { return false; }
    void await_suspend(std::coroutine_handle<> awaiter) {}
    void await_resume() {}
};

task myCoroutine() {
    // GCC: OK
    // clang: OK
    {
        co_await MoveOnlyAwaitable();
    }
    // GCC: OK
    // clang: OK
    {
        auto moveonly = MoveOnlyAwaitable();
        co_await std::move(moveonly);
    }

    // GCC <= 11.2: OK
    // GCC 11.3:ERROR:  error: use of deleted function
'MoveOnlyAwaitable::MoveOnlyAwaitable(const MoveOnlyAwaitable&)
    // clang: OK
    {
        auto moveonly = MoveOnlyAwaitable();
        co_await moveonly;
    }
}


Both gcc<11.3 and clang invoke MoveOnlyAwaitable&
task::promise_type::await_transform<MoveOnlyAwaitable&>(MoveOnlyAwaitable&) for
the last co_await, while GCC 11.3 and on seems to attempt to pass `moveOnly` by
value.

Manually instantiating the template, or creating a non-template overload that
accepts MoveOnlyAwaitable& doesn't work either.


I believe that this is a bug because calling

auto move_only = MoveOnlyAwaitable();
task::promise_type p;
p.await_transform(moveonly);

works even with GCC 11.3. As expected, the compiler produces the
await_transform<MoveOnlyAwaitable&>(MoveOnlyAwaitable&) overload, so it's weird
that it doesn't do so when `move_only` is passed to `co_await`.

[Bug c++/105406] coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[dvratil at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105406

--- Comment #1 from Daniel Vrátil <dvratil at kde dot org> ---
Link to godbolt: https://godbolt.org/z/MecdTEzMT

[Bug c++/105406] [11/12 Regression] coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105406

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.4
           Priority|P3                          |P2
           Keywords|                            |rejects-valid
      Known to work|                            |11.2.0
            Summary|coroutines: since 11.3      |[11/12 Regression]
                   |co_await attempts to copy a |coroutines: since 11.3
                   |move-only value when        |co_await attempts to copy a
                   |await_transform(T &) exists |move-only value when
                   |                            |await_transform(T &) exists
      Known to fail|                            |11.3.0, 12.0

[Bug c++/105406] [11/12 Regression] coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105406

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
                 CC|                            |iains at gcc dot gnu.org,
                   |                            |jakub at gcc dot gnu.org,
                   |                            |jason at gcc dot gnu.org
   Last reconfirmed|                            |2022-04-27

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Started with r12-618-g14ed21f8749ae359690d9c4a69ca38cc45d0d1b0
which has been backported to 11.3 in
r11-9055-gb874ece3ff95d3afa575d40b6e14e95cae8baf87

[Bug c++/105406] [11/12/13 Regression] coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105406

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:d0ed0690f1e86621a5cb62eec1d144036feb16f8

commit r13-6711-gd0ed0690f1e86621a5cb62eec1d144036feb16f8
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Mar 15 17:02:15 2023 -0400

    c++: co_await and move-only type [PR105406]

    Here we were building a temporary MoveOnlyAwaitable to hold the result of
    evaluating 'o', but since 'o' is an lvalue we should build a reference
    instead.

            PR c++/105406

    gcc/cp/ChangeLog:

            * coroutines.cc (build_co_await): Handle lvalue 'o'.

    gcc/testsuite/ChangeLog:

            * g++.dg/coroutines/co-await-moveonly1.C: New test.

[Bug c++/105406] [11/12 Regression] coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105406

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Jason Merrill
<jason@gcc.gnu.org>:

https://gcc.gnu.org/g:6fd32842404ac1a3cd98189f61d93c5bc9c8680c

commit r12-9435-g6fd32842404ac1a3cd98189f61d93c5bc9c8680c
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Mar 15 17:02:15 2023 -0400

    c++: co_await and move-only type [PR105406]

    Here we were building a temporary MoveOnlyAwaitable to hold the result of
    evaluating 'o', but since 'o' is an lvalue we should build a reference
    instead.

            PR c++/105406

    gcc/cp/ChangeLog:

            * coroutines.cc (build_co_await): Handle lvalue 'o'.

    gcc/testsuite/ChangeLog:

            * g++.dg/coroutines/co-await-moveonly1.C: New test.

[Bug c++/105406] [11 Regression] coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105406

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org

[Bug c++/105406] [11 Regression] coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105406

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jason Merrill
<jason@gcc.gnu.org>:

https://gcc.gnu.org/g:657fb0db2648a8cd7ba355fdec570fe2f08448ac

commit r11-10642-g657fb0db2648a8cd7ba355fdec570fe2f08448ac
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Mar 15 17:02:15 2023 -0400

    c++: co_await and move-only type [PR105406]

    Here we were building a temporary MoveOnlyAwaitable to hold the result of
    evaluating 'o', but since 'o' is an lvalue we should build a reference
    instead.

            PR c++/105406

    gcc/cp/ChangeLog:

            * coroutines.cc (build_co_await): Handle lvalue 'o'.

    gcc/testsuite/ChangeLog:

            * g++.dg/coroutines/co-await-moveonly1.C: New test.

[Bug c++/105406] [11 Regression] coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105406

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.4                        |11.5

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 11.4 is being released, retargeting bugs to GCC 11.5.

[Bug c++/105406] [11 Regression] coroutines: since 11.3 co_await attempts to copy a move-only value when await_transform(T &) exists

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105406

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
   Target Milestone|11.5                        |11.4
             Status|ASSIGNED                    |RESOLVED

--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed in 11.4/12.3/13.