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main.py

from typing import TypeVar, Generic, Any, cast
from abc import abstractmethod, ABC


_TP = TypeVar("_TP", bound=tuple[Any, ...])
_TM = TypeVar("_TM")

class Select(Generic[_TP]):
    pass


class Base:
    pass


class Other(Base):
    pass


ModelType = TypeVar("ModelType", bound=Base)


class DBRepository(ABC, Generic[ModelType]):
    
    T = TypeVar("T", Select[tuple[ModelType]], Select[tuple[int]])
    
    @abstractmethod
    def func(self, arg: T) -> T:
        pass


class SubDBRepository(DBRepository[Other]):
    def func(self, arg: Select[tuple[int]]) -> Select[tuple[int]]:
        return cast(Select[tuple[int]], Select())