Allow unsafe overrides in sub-subclasses that are compatible with subclass

Feature

When a subclass is implemented with unsafe method override, error can be suppressed with # type: ignore[override] comment. However, further subclassing requires this comment to be present in subclasses too.

Pitch

Suppose we have the following structure:

class A:
    def foo(self) -> int: pass

class B(A):
    def foo(self) -> str: pass  # type: ignore[override]

class C(B):
    def foo(self) -> str: pass

Currently mypy complains:

example.py:8: error: Return type "str" of "foo" incompatible with return type "int" in supertype "A"

But in reality we do expect that all B subclasses should implement foo with signature () -> str. For instance, it is useful when B is replacement for A with slightly different usage (when we don't want to replicate all internal logic - e.g. if it comes from third-party package). It takes care to override all methods that became incompatible after this change, and now B is consistent. Then we want to add some feature to B via subclassing while preserving its interface. All signatures are preserved, but mypy complains that we are incompatible with old base (A). I think that when unsafe override is explicitly ignored once, mypy should check new implementation against definitions in B, not in A, ignoring the fact that B has unsafe overrides. Actually now the checker reports errors that are explicitly ignored: we already took care to ignore that errors inside B.

Real world example: ModelChoiceField in Django inherits from ChoiceField (yeah, that's bad design decision, but I'm working on stubs and can't change the source). Incompatible part:

class ChoiceField(Field):
    def to_python(self, data: Optional[Any]) -> str: ...

class ModelChoiceField(ChoiceField):
    def to_python(self, data: Optional[Any]) -> Optional[Model]: ...  # type: ignore[override]

When end Django user tries to subclass ModelChoiceField and implements to_python with same signature, user receives mypy error. However, this implementation is perfectly valid as long as direct superclass already defines that incompatible change.

L'issue ne nomme aucun fichier source ni aucun test. Commencez par reproduire l'exemple A/B/C et suivez la vérification des overrides de mypy pour les méthodes des sous-classes ; c'est terminé lorsque l'override compatible de C est accepté en se basant sur B, tandis que l'incompatibilité de B, explicitement ignorée, reste supprimée.