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

Start with the reproduction in mypytest.py and run it with mypy to confirm the inferred type and error. Trace how the type of a list of classes with multiple shared bases is inferred, then verify that both b1 and b2 are accepted without losing the multiple-base relationship.