`ClassVar` cannot be overridden with `Final`

[romintomasetti]

Bug Report

A typing.ClassVar cannot be overridden with typing.Final.

main.py:7: error: Cannot override class variable (previously declared on base class "Base") with instance variable  [misc]
Found 1 error in 1 file (checked 1 source file)

To Reproduce

import typing

class Base:
    value : typing.ClassVar[str]
    
class Derived(Base):
    value : typing.Final[str] = 'derived'

https://mypy-play.net/?mypy=latest&python=3.12&flags=verbose&gist=03819c6e8c7b6fb53baebe094faed123

Expected Behavior

I though using a typing.Final annotation would imply typing.ClassVar.

Actual Behavior

It seems not allowed to override a ClassVar with a Final.

Your Environment

See reproducer.

[JelleZijlstra]

I believe this is correct because a ClassVar is mutable, so it would be possible to do something like t: type[Base]; t.value = "x".

[sterliakov]

This violates LSP: Base.value = "foo" is a valid assignment, as is t: type[Base]; t.value = "foo", so Derived is not a valid type[Base], this should not be allowed.

The error message is confusing, though. The problem is indeed mutability, not Final creating an "instance variable" - it is assigned in class body, it is still class-level. Final implies ClassVar everywhere except dataclasses.