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From: "lemourin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/99576] New: [coroutines] desctructor of a temporary called
 too early within co_await expression
Date: Sat, 13 Mar 2021 13:54:16 +0000
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X-Bugzilla-Version: 10.2.1
X-Bugzilla-Keywords: wrong-code
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99576

            Bug ID: 99576
           Summary: [coroutines] desctructor of a temporary called too
                    early within co_await expression
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lemourin at gmail dot com
  Target Milestone: ---

```
#if __has_include(<coroutine>)
#include <coroutine>
namespace stdx {
using std::coroutine_handle;
using std::noop_coroutine;
using std::suspend_always;
using std::suspend_never;
}  // namespace stdx
#elif __has_include(<experimental/coroutine>)
#include <experimental/coroutine>
namespace stdx {
using std::experimental::coroutine_handle;
using std::experimental::noop_coroutine;
using std::experimental::suspend_always;
using std::experimental::suspend_never;
}  // namespace stdx
#else
#error "coroutines unsupported"
#endif
#include <exception>
#include <iostream>
#include <utility>

class Task {
 public:
  struct promise_type {
    struct final_awaitable {
      bool await_ready() noexcept { return false; }
      auto await_suspend(stdx::coroutine_handle<promise_type> coro) noexcep=
t {
        return coro.promise().continuation;
      }
      void await_resume() noexcept {}
    };
    Task get_return_object() {
      return Task(stdx::coroutine_handle<promise_type>::from_promise(*this)=
);
    }
    stdx::suspend_always initial_suspend() { return {}; }
    final_awaitable final_suspend() noexcept { return {}; }
    void unhandled_exception() { std::terminate(); }
    void return_void() {}

    stdx::coroutine_handle<void> continuation =3D stdx::noop_coroutine();
  };

  Task(const Task&) =3D delete;
  Task(Task&& other) noexcept
      : handle_(std::exchange(other.handle_, nullptr)) {}
  Task& operator=3D(const Task&) =3D delete;
  Task& operator=3D(Task&& other) noexcept {
    handle_ =3D std::exchange(other.handle_, nullptr);
    return *this;
  }
  ~Task() {
    if (handle_) {
      handle_.destroy();
    }
  }

  bool await_ready() const { return false; }
  auto await_suspend(stdx::coroutine_handle<void> continuation) {
    handle_.promise().continuation =3D continuation;
    return handle_;
  }
  void await_resume() {}

 private:
  explicit Task(stdx::coroutine_handle<promise_type> handle)
      : handle_(handle) {}

  stdx::coroutine_handle<promise_type> handle_;
};

struct RunTask {
  struct promise_type {
    RunTask get_return_object() { return {}; }
    stdx::suspend_never initial_suspend() { return {}; }
    stdx::suspend_never final_suspend() noexcept { return {}; }
    void return_void() {}
    void unhandled_exception() { std::terminate(); }
  };
};

struct Foo {
  Foo() { std::cerr << "Foo()\n"; }
  ~Foo() { std::cerr << "~Foo()\n"; }
};

Task DoAsync() {
  std::cerr << "START TASK\n";
  co_return co_await [foo =3D Foo{}]() -> Task {
    std::cerr << "IN LAMBDA\n";
    co_return;
  }();
}

RunTask Main() { co_await DoAsync(); }

int main() { Main(); }
```

gcc outputs:
`
START TASK
Foo()
~Foo()
IN LAMBDA
`

while clang and msvc output:
`
START TASK
Foo()
IN LAMBDA
~Foo()
`

Is this code undefined behavior? Or is clang and msvc wrong?=