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

mypytest.py の再現コードから始め、mypy で実行して推論された型とエラーを確認します。複数の共通基底クラスを持つクラスのリストの型がどのように推論されるかを追跡し、その後、複数の基底クラスの関係を失うことなく b1 と b2 の両方が受け入れられることを確認します。