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`.
