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]

Issue にある最小化された Python の再現コードから始め、mypy を実行して reveal_type が datetime64[Any] を報告することを確認します。結果を、リンクされた pyright の例にある期待される datetime64[int | None] と比較します。mypy がこの期待どおりにジェネリックのデフォルト型を一貫して解決し、回帰テストのカバレッジが追加されれば完了です。