Generic default type in `type` revealed as `Any`

Bug Report

mypy reveals a certain nested default generic type in type as Any, while other type checkers reveal it as the default type.

This issue is motivated by pandas-dev/pandas-stubs#1764.

To Reproduce

This is complicated, see https://github.com/python/mypy/issues/21608#issuecomment-4692751812 for a much simplified example.

from typing import Any, Generic, TypeVar, reveal_type

_ItemT_co = TypeVar("_ItemT_co", default=Any, covariant=True)

class generic(Generic[_ItemT_co]): ...

_ScalarT_co = TypeVar("_ScalarT_co", bound=generic, default=Any, covariant=True)

class dtype(Generic[_ScalarT_co]): ...

_DT64ItemT_co = TypeVar(
    "_DT64ItemT_co", bound=int | None, default=int | None, covariant=True
)

class datetime64(generic[_DT64ItemT_co], Generic[_DT64ItemT_co]): ...

GenericT_co = TypeVar("GenericT_co", bound=generic, default=Any, covariant=True)
GenericT = TypeVar("GenericT", bound=generic, default=Any)

class Series(Generic[GenericT_co]):
    def to_numpy_simplified(
        self, dtype: dtype[GenericT] | type[GenericT]
    ) -> GenericT: ...

reveal_type(Series().to_numpy_simplified(datetime64))  # mypy: datetime64[Any], pyright & pyright: datetime64[int | None]

• https://gist.github.com/mypy-play/099705d9f12556f66776949caa6f0450
• https://pyright-play.net/?strict=true&code=GYJw9gtgBALgngBwJYDsDmUkQWEMoCCKcANFAOICmKlISAxmQCqKUBqAhiGSJQG6UOAGwD68BJQCwAKBkiAkjEoQmI%2BmCgBeKCwmcQACgBECpSrVgjZACaVgHAK5CYmoqSjq%2BXJBxQumIA6UAJQyMvRCHADOUVBo1LQMBlQ0dPQA2qbKquoAusEAXFAAdKVh0iIAyvTCXDka2rrsXMZVNZEg9VZQAEZgDijWmvGpDDZ2js6uxGSe3r7%2BgSHlEdGx1uKUyQlpmdW1nRb5RaXF5SIAIkwAbAAsitkWWjqs%2BgYyUJ9QJld3D%2Bbqbp9AZDVD4AA%2BUAAcmAaON7E4XGCoJCYXCPGAvHQFpoAkEZKFZNJVjEoNYOEoYFhKHcDCNEhlLjd7mZ6rkyCkGZlfizHnlCiUykTOWl6s8mm8jCKGF0yMDBsMdmMyRNEdN3HNsX5cUtCdL6Exxa8WlKlQagf0FfS0vDJi43ITwpFSZVEpQottRhl9WzCh8vrZgLAwCIUA5sHARFEsAghEhgEhKNZ3tIvmmoFFKEJgDZNkUNqx0j7cijYIXi-7PsEoABaAB8FDNTBOQpkvAEwjErAMrro7oMwWKMBDYYjUZjcYTSYM5Mp1LuwUJQA
• https://play.ty.dev/8c070d59-a962-4cd0-823f-fa8dd9941f4f

Expected Behavior

datetime64[int | None]

Actual Behavior

datetime64[Any]

Comienza con la reproducción minimizada en Python del issue y ejecuta mypy para confirmar que reveal_type informa datetime64[Any]. Compara el resultado con el datetime64[int | None] esperado de los ejemplos de pyright enlazados; se considera terminado cuando mypy resuelve de forma coherente el tipo genérico predeterminado de acuerdo con esa expectativa y se añade cobertura de regresión.