Shared via mypy Playground

main.py

from __future__ import annotations

import typing as t


_ST = t.TypeVar("_ST")
_GT = t.TypeVar("_GT")


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


_ST_CharField = t.TypeVar("_ST_CharField", default=str | int)
_GT_CharField = t.TypeVar("_GT_CharField", default=str)


class CharField(Field[_ST_CharField, _GT_CharField]):
    @t.overload
    def __new__(
        cls, *, null: t.Literal[True]
    ) -> CharField[_ST_CharField | None, _GT_CharField | None]: ...

    @t.overload
    def __new__(
        cls, *, null: t.Literal[False] = False
    ) -> CharField[_ST_CharField, _GT_CharField]: ...

    def __new__(   # type: ignore[misc]
        cls, *, null: bool = False
    ) -> (
        CharField[_ST_CharField, _GT_CharField]
        | CharField[_ST_CharField | None, _GT_CharField | None]
    ):
        return super().__new__(cls)


x = CharField(null=False)
reveal_type(x)
y = CharField(null=True)
reveal_type(y)
z = CharField()
reveal_type(z)