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main.py

import typing as tp
if not tp.TYPE_CHECKING:
    reveal_type = lambda var: var


class Foo:  # Only supports "*Foo*".
    def __init__(self, value: int) -> None:
        self.value = value
    
    def __add__(self, other: tp.Union["Foo", "FooBar"]) -> "Foo":
        if not isinstance(other, (Foo, FooBar)):
            return NotImplemented
        self.value += other.value
        return self

    def __repr__(self) -> str:
        return "Foo({})".format(self.value)


class FooBar:  # Supports all three.
    def __init__(self, value: int) -> None:
        self.value = value

    def __add__(self, other: tp.Union["FooBar", Foo, "Bar"]) -> "FooBar":
        if not isinstance(other, (FooBar, Foo, Bar)):
            return NotImplemented
        self.value += other.value
        return self

    def __repr__(self) -> str:
        return "FooBar({})".format(self.value)


class Bar:  # Only supports "*Bar*".
    def __init__(self, value: int) -> None:
        self.value = value
    
    def __add__(self, other: tp.Union["Bar", FooBar]) -> "Bar":
        if not isinstance(other, (Bar, FooBar)):
            return NotImplemented
        self.value += other.value
        return self

    def __repr__(self) -> str:
        return "Bar({})".format(self.value)


Fooable = tp.Union[FooBar, Foo]
Barable = tp.Union[Bar, FooBar]
T1 = tp.TypeVar("T1", Fooable, Barable)


def add_foo_bar(a: T1, b: T1, c: T1) -> T1:
    return a + b + c


if __name__ == "__main__":
    # Should resolve to 'Fooable' because arguments don't contain 'Bar'.
    print(reveal_type(add_foo_bar(FooBar(1), Foo(2), FooBar(3))))
    print(reveal_type(add_foo_bar(FooBar(1), Bar(3), FooBar(-2))))  # Should resolve to 'Barable'.
    print(reveal_type(add_foo_bar(Bar(1), Foo(2), Foo(3))))  # Should emit type-check & runtime error.