list of types with multiple bases only uses first shared base?

i may be misunderstanding something, but it seems that when making a list of types, each with multiple shared base classes, mypy mistakenly infers this to be a list of the first base only, missing others

$ cat mypytest.py
# two different base classes with different attributes
class B1:
    b1 = None


class B2:
    b2 = None


# two classes that subclass both B1 and B2
class S1(B1, B2):
    pass


class S2(B1, B2):
    pass


# a list of the subclasses
types = [S1, S2]

# it seems mypy thinks `types` is List[Type[B1]]
ok = [x.b1 for x in types]
why_not_ok = [x.b2 for x in types]

$ mypy mypytest.py
mypytest.py:24: error: "Type[B1]" has no attribute "b2"

$ mypy --version
mypy 0.770+dev.b2edab25b7e4a6131484bb6edf314e2fa078aee7
$ python -V
Python 3.7.3

Commencez par la reproduction dans mypytest.py et exécutez-la avec mypy pour confirmer le type inféré et l’erreur. Suivez la manière dont est inféré le type d’une liste de classes ayant plusieurs classes de base communes, puis vérifiez que b1 et b2 sont tous deux acceptés sans perdre la relation entre les multiples classes de base.