@Yury: depends on what you mean by "100% reliable" :-). Like I said above, I'm normally super against automagic detection of sync-vs-async functions because of all the edge cases where it goes wrong, but in this specific case where people are writing a decorator one line above their def/async def, I think a simple iscoroutinefunction check will be pretty close to 100% matching what users expect.

Or, if we don't change the semantics, then we can still be 100% confident that if iscoroutinefunction returns true, then the user has made a mistake. (I.e., if we make this issue a warning, then it's possible we'll miss print a warning in some complicated cases, but we can be confident that all the warnings we do print are correct.)