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.

まず issue の最小再現を実行し、AppClass1 と AppClass2 の診断結果を比較します。mypy が Composite からの継承と B1 および B2 にある互換性のない foo の定義をどのように処理するかを追跡します。動作を一貫して理解し、回帰テストでカバーできれば完了です。