Shared via mypy Playground

main.py

from typing import TypeVar, Literal, Generic, overload, reveal_type
from typing import Any

_ST = TypeVar("_ST")
_GT = TypeVar("_GT")

class Field(Generic[_ST, _GT]):
    def __init__(self, *, null: bool = False) -> None:
        super().__init__()

    def __get__(self, instance: object, owner: object) -> _GT:
        ...

_ST_1 = TypeVar("_ST_1", default=int)
_GT_1 = TypeVar("_GT_1", default=int)

class IntegerField(Field[_ST_1, _GT_1]):
    @overload
    def __new__(cls, *, null: Literal[True]) -> "IntegerField[_ST_1 | None, _GT_1 | None]": ...
    @overload
    def __new__(cls, *, null: Literal[False] = False) -> "IntegerField[_ST_1, _GT_1]": ...
    def __new__(cls, *, null: bool = False) -> "IntegerField[Any, Any]":
        return super().__new__(cls)
class Model:
    pass


class A(Model):
    field1 = IntegerField(null=False)
    field2 = IntegerField(null=True)

a = A()
reveal_type(a.field1)  # expect: int
reveal_type(a.field2)  # expect: int | None