mypy incorrectly allows overriding parent attribute

[jfgauron]

Bug Report

mypy permits overriding attributes of parent class if the child attribute is compatible with the parent attribute. This breaks Liskov Substitution Principle.

To Reproduce

from typing import Literal

class Parent:
  x: Literal["a", "b", "c"]

  def __init__(self, x: Literal["a", "b", "c"]) -> None:
    self.x = x

class Child(Parent):
  x: Literal["a"]

  def foo(self) -> Literal["a"]:
    return self.x

child = Child("c") # no error from mypy
x = child.foo()

# reveal_type(x) # Revealed type is "Literal['a']
print(x) # outputs "c"

https://mypy-play.net/?mypy=latest&python=3.11&gist=37e7f5a43c649a9ea5d6f0a9253d7e07

Expected Behavior

I believe mypy shouldn't allow overriding class attributes in child class, even with a type that is "compatible" with the parent attribute. Otherwise we can get into funny cases such as:

my_list: list[Parent] = [Child("a")]
my_list[0].x = "c"

Actual Behavior

I got no error from mypy. Running python -m mypy --strict main.py printed:

Success: no issues found in 1 source file

Your Environment

• Mypy version used: 1.3.0
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10.6

[hauntsaninja]

Duplicate of #3208

I think the dropbox folks found this was too strict to have on by default in practice. mypy is strict about this with protocols (which is good) and folks still file issues about that somewhat regularly. Anyway that^ issue is where to discuss