mypy is probably being too smart and seeing that if current.__cause__ is not None is always True so chained_msg is always a str and not str | None. This will probably be the case until the linked PR is merged. I can explicitly type out with list[tuple[str | None, traceback.TracebackException]] for now if desired.

@davidism would it be possible to review this now that the above PR is merged?

Thanks. I've merged it manually to stable as the change of base here didn't work as I'd hoped.