Let's start with a couple of base classes in a classical diamond-inheritance shape, with member conflict explicitly solved:

class A:
    class M:
        pass

class B0(A):
    class M(A.M):
        pass

class B1(A):
    class M(A.M):
        pass

class C(B0,B1):
    class M(B0.M, B1.M):
        pass

All of these in real code are abstract base classes, classes A , B0, B1 all add behaviors of their own; class C is here precisely to provide this version of M compatible with both B0 and B1 and deal with diamond inheritance in a "centralized" way, so all classes deriving from B0 and B1 don't have to duplicate the same version of M over and over. It could, though, also add behavior of its own.

More abstract classes derive from B0 and B1, each adding more behavior:

class D0(B0):
    pass
class D1(B1):
    pass

And then comes concrete class deriving from D0 and D1, deriving from C to get the diamond inheritance solved:

class D(D0,D1,C):
    pass

python has no issue with this, and understands D.M as defined as C.M. mypy on the other hand does not take python's resolution of the problem into account, and compares all bases classes, leading to false positive:

test-mypy-diamond-inheritance.py:22: error: Definition of "M" in base class "B0" is incompatible with definition in base class "B1"  [misc]

To Reproduce

See https://mypy-play.net/?gist=132dcea42366fad6d56d9c150d8e6c60