Error deriving from a composite of two incompatible base classes

Simple repro:

class B1:
    def foo(self) -> int:
        ...

class B2:
    def foo(self) -> str:
        ...

class Composite(B1, B2):  # type: ignore
    ...

class AppClass1(Composite):  # <-- no error here
    ...

class AppClass2(Composite, object):  # <-- error here!
    ...

In the real world case, classes B1, B2 and Composite are part of a library (PEP-561-compliant), and classes AppClass1 and AppClass2 are part of the application code that's being type-checked.

The surprising part is that while AppClass2's declaration causes an error (even though AppClass2 didn't introduce the incompatibility), this error isn't seen in AppClass1's declaration.

Start by running the minimal reproduction in the issue and compare diagnostics for AppClass1 and AppClass2. Trace how mypy handles inheritance from Composite and the incompatible foo definitions in B1 and B2; done means the behavior is consistently understood and covered by a regression test.