`__slots__` is incorrectly classified as an instance variable across inheritance

[XuehaiPan]

Bug Report

Mypy classifies __slots__ as an instance variable even though Python defines it on the class. This causes a false-positive override error when a subclass explicitly annotates __slots__ as a ClassVar.

The issue also affects plugin-generated __slots__, including slots generated by dataclasses and attrs.

To Reproduce

from dataclasses import dataclass
from typing import ClassVar


@dataclass(slots=True)
class Base:
    x: float
    y: float


class Child(Base):
    __slots__: ClassVar[tuple[()]] = ()

Run:

mypy example.py

Playground link: https://mypy-play.net/?mypy=latest&python=3.14&gist=e0e2f906ab9e0201c43d2b5cb6ff950c

Expected Behavior

Mypy should report no errors. Both generated and explicitly declared __slots__ should be treated as class variables across inheritance.

Actual Behavior

Cannot override instance variable (previously declared on base class "Base") with class variable  [misc]

A # type: ignore[misc] comment is currently required on the subclass declaration even though __slots__ is a class-level attribute.

The same inconsistent classification occurs with ordinary __slots__ declarations and attrs-generated slots.

Your Environment

• Mypy version used: 2.3.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.14.7

[bzoracler]

__slots__ is only useful as effectively a ClassVar[], that I agree with. But it is not a type error to treat it as an instance variable. You can arbitrarily assign __slots__ at the instance level, no matter how weird it may look; the following works at runtime and mypy won't emit diagnostics:

class A:
    __slots__ = ("__dict__",)

a = A()
a.__slots__ = ("hello",)

Like the implicitly-generated __slots__ by @dataclass(slots=True), the error Cannot override instance variable with class variable is the same as any other code which doesn't annotate ClassVar[] in a base class:

from typing import ClassVar

class Base:
    a = 1

class Child(Base):
    a: ClassVar[int] = 2  # Cannot override instance variable (previously declared on base class "Base") with class variable  [misc]

If mypy emits diagnostics for @dataclass(slots=True) and incompatible child ClassVar[] annotations, it would be a linting rule rather than a type error. I have no real opinion on this as __slots__ is special-cased to allow any Iterable[str] annotation in sub/superclass relations in an LSP-incompatible manner, so I've never added type annotations for __slots__ anyway. Some people may want to annotate this if they're using the value of ConcreteClass.__slots__ directly (dangerously LSP-incompatible though if ConcreteClass: type[BaseClass]), so I guess it may benefit them.