Shared via mypy Playground

main.py

from typing import Generic, Any, overload
from typing_extensions import TypeVar

class C:
    pass

class CustomC(C):
    pass

_C = TypeVar('_C', bound=C, default=C)

class A(Generic[_C]):
    @overload
    def __init__(self: 'A[C]') -> None: ...
        
    @overload
    def __init__(self: 'A[_C]', c: type[_C]) -> None: ...
    
    def __init__(self, c: Any = C) -> None:
        self._c: type[_C] = c

    @property
    def c(self) -> type[_C]:
        return self._c

    @c.setter
    def c(self, c: type[_C]) -> None:
        self._c = c
        
plain = A()
reveal_type(plain)
reveal_type(plain.c)

custom = A(CustomC)
reveal_type(custom)
reveal_type(custom.c)