Function impl_

3rd/asio-1.24.0/include/asio/experimental/promise.hpp:145–147

  std::shared_ptr<detail::promise_impl<void(Ts...), Executor>> impl_;
  promise(std::shared_ptr<detail::promise_impl<void(Ts...), Executor>> impl)
    : impl_(impl)
  {
  }

  struct initiate_async_wait
  {