Shouldn't it instead say: "consider addingasync todef lauch()"?

Because in my experience, AsyncIterators are way more common withasync def rather than regulardef

Ehh, this is confusing :|

The 'problem' here should really be reported on this line:

  class P(Protocol):!>    async def launch(self) -> AsyncIterator[int]:         raise BaseException

but we don't put notices proactively on declarations that smell funny, or do we?

The error on the derivedLauncher.launch gives us the excuse to make a notice. What we want to say is "listen, we must error onLauncher.launch, butLauncher.launch is probably fine, consider changing the base declaration instead".