Function async_answer

examples/hello_coro.cpp:26–31

template <sender S1, sender S2>
auto async_answer(S1 s1, S2 s2) -> stdexec::task<int>
{
  // Senders are implicitly awaitable (in this coroutine type):
  co_await static_cast<S2&&>(s2);
  co_return co_await static_cast<S1&&>(s1);
}

template <sender S1, sender S2>
auto async_answer2(S1 s1, S2 s2) -> stdexec::task<std::optional<int>>