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main.py

from __future__ import annotations
from abc import abstractmethod
from typing import Protocol, TypeVar

T = TypeVar("T")
T_add = TypeVar("T_add", bound="Addable")


class Addable(Protocol):
    
    def __add__(self: T, other: T) -> T:
        pass


def foo(x: T_add, y: T_add) -> T_add:
    return x + y



myint: int = foo(1, 2)
mylist: list[int] = foo([1], [2])
mydict: dict = foo({"a": 1}, {"b": 2})

This seems to work - myint and mylist are accepted, mydict is rejected for the right reason.

(You can put type hint dict[str, int] on mydict to silence a different error)