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main.py

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


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

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]])
    
    @overload
    def func(self, arg: Select[tuple[ModelType]]) -> Select[tuple[ModelType]]:
        ...
    @overload
    def func(self, arg: Select[tuple[int]]) -> Select[tuple[int]]:
        ...
    @abstractmethod
    def func(self, arg):
        ...

class SubDBRepository(DBRepository[Other]):
    @overload
    def func(self, arg: Select[tuple[Other]]) -> Select[tuple[Other]]:
        ...
    @overload
    def func(self, arg: Select[tuple[int]]) -> Select[tuple[int]]:
        ...
    def func(self, arg):
        # here we are runtime, runtime cannot distinguish
        # between tuple[ModelType]() and tuple[int](); can
        # only see a difference if the arg is a non-empty tuple
        if len(arg):
            match arg[0]:
                case Other():
                    # need to come up with a Select[tuple[Other]] but for now:
                    return arg
                case int():
                    # need to come up with a Select[tuple[int]] but for now:
                    return arg
        else:
            return Select()