results

$ mypy test.bad.py
test.bad.py:22: error: "V2" has no attribute "method"

$ mypy /tmp/test.good.py
[no errors returned]

test.bad.py

import typing

VectorLike = typing.Union[
    'V2',
    typing.Tuple[int, int],
]


class V2:
    def __init__(self, x: int, y: int) -> None:
        pass

    def __add__(self, other: VectorLike) -> V2:
        return self


class V(V2):
    def method(self) -> typing.NoReturn:
        raise NotImplementedError

x = V(1, 0) + (0, 1)
x.method()

test.good.py

from numbers import Real
import typing


VectorLike = typing.Union[
    'V2',
    typing.Tuple[int, int],
]


class V2:
    def __init__(self, x: int, y: int) -> None:
        pass

    T = typing.TypeVar('T', bound=V2)
    def __add__(self: T, other: VectorLike) -> T:
        return self


class V(V2):
    def method(self) -> typing.NoReturn:
        raise NotImplementedError

x = V(1, 0) + (0, 1)
x.method()