Specialized methods on generic classes

Bug Report

Let's say I have a generic class G.

from __future__ import annotations

from collections.abc import Sized
from typing import Generic, TypeVar

T = TypeVar("T")

class G(Generic[T]):
    pass

I want to have a single method on this class that's only legal for a subset of T.

If I want to limit it to, say, int, it works:

class G(Generic[T]):
    def test(self: G[int]) -> G[int]:
        return self

G[int]().test()  # Works, as it should
G[str]().test()  # Error, as it should

But if I want to limit it to, say, a protocol like Sized, I get a false negative:

S = TypeVar("S", bound=Sized)

class G(Generic[T]):
    def test(self: G[S]) -> G[S]:
        return self


G[int]().test()  # False negative - works, but it shouldn't

This is a pity, since it'd unlock some cool features I was working on vis-a-vis using attrs attributes. Are there any workarounds? Would this be handled by intersection types maybe?

Your Environment

• Mypy version used: mypy 1.0.0+dev.b8c03ab6809aab56928f3cd865edb44944a600a2
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: Python 3.10.6

Commencez par exécuter le reproducteur Python fourni avec la version de mypy indiquée et comparez le comportement de la spécialisation int avec celui de la spécialisation contrainte par Sized. Suivez le chemin de vérification du self-type générique, puis ajoutez ou mettez à jour un test de régression afin que les méthodes spécialisées contraintes par un protocole soient acceptées ou rejetées conformément au comportement attendu.