Small issues.

[qis]

I'll cram everything I find while modifying your async_generator implementation to meet my personal needs in here. Numbers, so that it's easier to answer, if you choose to do so. Hope this helps.

1. Is your code licensed or licensable?

2. Shouldn't the move op also destroy the coroutine since the async_generator owns it?

async_generator& operator=(async_generator&& other) {
  // _Coro was renamed to handle_ in my code.
  if (&other != this) {
    if (handle_) {
      handle_.destroy();
    }
    handle_ = std::exchange(other.handle_, nullptr);
  }
}

3. You're not using the await_iterator::owner boolean anymore.

4. Inheriting from std::iterator is deprecated, if I'm not mistaken.

using iterator_category = std::input_iterator_tag;
using difference_type = std::ptrdiff_t;
using value_type = T;
using pointer = T*;  // I needed an iterator instead of a const_iterator
using reference = T&;

5. According to VS 15.4.2 warnings, from_promise taking a pointer is deprecated and accepts the promise by reference.

P.S.: Your code helped me a lot. Thank you! :)