Shared via mypy Playground

main.py

from typing import (
    Generic,
    Literal,
    TypeAlias,
    overload,
)

from typing_extensions import (
    TypeVar,
)

Storage: TypeAlias = Literal["python", "pyarrow"]
StorageT = TypeVar("StorageT", bound=Storage)
_StorageT = TypeVar("_StorageT", bound=Storage | None, default=None)

class _StringDtypeStorageDescriptor:
    @overload
    def __get__(
        self, instance: StringDtype[None], owner: type[StringDtype[None]]
    ) -> Storage: ...
    @overload
    def __get__(
        self, instance: StringDtype[StorageT], owner: type[StringDtype[StorageT]]
    ) -> StorageT: ...

class StringDtype(Generic[_StorageT]):
    storage = _StringDtypeStorageDescriptor()

reveal_type(StringDtype[Literal["python"]]().storage)  # mypy, pyright, ty: Literal["python"], pyrefly: error