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main.py

from typing import TypeVar, Generic, ClassVar, Self

T = TypeVar("T", bound="BaseModel")

class BaseModel:
    _meta: "ClassVar[Meta[Self]]"

    class Meta(Generic[T]):
        def __init__(self, model: type[T]) -> None:
            self.model = model

    @classmethod
    def __init_subclass__(cls) -> None:
        cls._meta = cls.Meta(cls)

reveal_type(BaseModel._meta)