minimal reproduction: ```python import gzip import tarfile open('t', 'w').close() with gzip.GzipFile('out.tar.gz', 'wb') as gzipf: with tarfile.open(fileobj=gzipf, mode='w') as tf: tf.add('t') ``` ```console $ python3 t.py $ mypy t.py t.py:7: error: Argument "fileobj" to "open" has incompatible type "GzipFile"; expected "Optional[IO[bytes]]" Found 1 error in 1 file (checked 1 source file) $ mypy --version mypy 0.812 ``` presumably `fileobj` should use a `Protocol` of some shape ?