[Bug c++/99575] New: [coroutines] unexpected move when co_awaiting a nonmoveable object

["lemourin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99575

            Bug ID: 99575
           Summary: [coroutines] unexpected move when co_awaiting a
                    nonmoveable object
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Keywords: rejects-valid
          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

class Task {
 public:
  struct promise_type {
    Task get_return_object() { return Task{}; }
    stdx::suspend_always initial_suspend() { return {}; }
    stdx::suspend_always final_suspend() noexcept { return {}; }
    void unhandled_exception() {}
    void return_void() {}
  };

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

class NonMoveableTask {
 public:
  NonMoveableTask() = default;
  NonMoveableTask(const NonMoveableTask&) = delete;
  NonMoveableTask(NonMoveableTask&&) = delete;

  NonMoveableTask& operator=(const NonMoveableTask&) = delete;
  NonMoveableTask& operator=(NonMoveableTask&& other) = delete;

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

Task Foo(NonMoveableTask* task) { co_await* task; }

int main() {}
```

The above code snippet does not compile with gcc with the following error log:
`
move.cc: In function ‘Task Foo(NonMoveableTask*)’:
move.cc:50:45: error: use of deleted function
‘NonMoveableTask::NonMoveableTask(NonMoveableTask&&)’
   50 | Task Foo(NonMoveableTask* task) { co_await* task; }
      |                                             ^~~~
move.cc:40:3: note: declared here
   40 |   NonMoveableTask(NonMoveableTask&&) = delete;
      |   ^~~~~~~~~~~~~~~
make: *** [<builtin>: move] Error 1
`

clang and msvc accept it. If the move constructor is available code compiled
with gcc uses it and leaves `task` in a moved from state.