Yeah, it only becomes an issue if you want the compiler to infer that ImpossibleError cannot be raised in

    while True: pass
    raise ImpossibleError

But normally we are perfectly content to put ImpossibleError in the signature for code like this.

Actually in this specific case you can infer that, and you can have an exhaustive list of language constructs beyond which this becomes undecidable.

There is a practical example of “panic!” in Rust to go around that as well.