Allow a base class to restrict the type of subclass's inner class

Feature

For regular classvars, attributes and methods, mypy already checks that the types in a subclass match those declared in the superclass. However, this checking does not extend to inner classes in the subclass. It seems inner classes are ignored by mypy entirely.

For example, a framework could define an abstract class Model whose inner class Meta must implement a certain protocol:

# Defined in the framework
class InnerMetaProtocol(Protocol):
    example_attr: str

class Model:
    Meta: type[InnerMetaProtocol]


# An application developer using framework
class Widget(Model):
    class Meta:
        example_attr = 123  # This should be an error: int is not str

Class Widget implements Model, whose attribute Meta should conform to type[InnerMetaProtocol], meaning it requires attribute example_attr: str. But currently mypy ignores this constraint: Playground.

It's probably useful to support this with not just protocols, but also concrete classes (e.g. Meta: type[ConcreteClass]).

Pitch

This pattern of inner Meta classes is used quite extensively in the Django ecosystem, including for Django models, forms, Django REST Framework serializers, djangorestframework-dataclasses, etc. I'm sure there are other use cases as well, but I don't know off the top of my head.

Inizia con l'esempio Playground della issue e confronta il tipo dichiarato di Model.Meta con la classe interna Meta della sottoclasse. Il lavoro è completato quando mypy segnala l'esempio incompatibile con int e supporta i casi indicati del protocollo e della classe concreta.