sys.stdout.buffer has incorrect type?

[vlovich]

mypy test.py
test.py:10: error: Argument 1 to "test" has incompatible type "BinaryIO"; expected "BufferedIOBase"

cat test.py

import io
import sys

class C(io.BufferedIOBase):
  pass

def test(c: io.BufferedIOBase):
  assert isinstance(c, io.BufferedIOBase)

test(sys.stdout.buffer)
test(C())

python3 test.py

As we can see the isinstance check passes but mypy asserts

[srittau]

This is currently hard to fix and depends on typeshed using protocols for file argument types. See python/typing#564 for more information.