[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" <gcc-bugzilla@gcc.gnu.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`.