Using type Self allows to violate Liskov Substitution Principle

Bug Report

By using Self in the type of a method's argument it's possible to violate the Liskov Substitution Principle without any warning.

(A clear and concise description of what the bug is.)

To Reproduce

Mypy raises no issue on this code although SubClass.update is clearly violating LSP.

from typing_extensions import Self

class BaseClass:
    foo: int
    def update(self, other: Self) -> None:
        self.foo = other.foo

class SubClass(BaseClass):
    bar: int
    def update(self, other: Self) -> None:  # problem here
        super().update(other)
        self.bar = other.bar

Expected Behavior

I'd expect that using Self would raise the warning similarly to specifying the class. Mypy raises the issue on the below.

from typing_extensions import Self

class BaseClass:
    foo: int
    def update(self, other: BaseClass) -> None:
        self.foo = other.foo

class SubClass(BaseClass):
    bar: int
    def update(self, other: SubClass) -> None:  # problem here
        super().update(other)
        self.bar = other.bar

Your Environment

• Mypy version used: 1.4.1
• Python version used: 3.10.8

Inizia eseguendo il riproduttore Python nell’issue e confronta l’override basato su Self con le annotazioni esplicite di BaseClass e SubClass. Traccia il controllo degli override di mypy per i parametri dei metodi che coinvolgono Self, quindi aggiungi la copertura di regressione per il caso segnalato. Il lavoro è completato quando mypy segnala l’override non valido preservando al contempo l’uso valido di Self.