calls promise.return_value(value), for 'co_return expr();',
calls promise.return_void(), for 'co_return;' (or simply reaching the end of body),
calls promise.unhandled_exception() if it terminates by throwing an exception that it doesn't itself catch (notably, the exception doesn't fall out of the coroutine to the resumer).
In the implementation of these, you'd want to save the return value (or the exception info) somewhere, to be retrieved later.
alcinnz