Protocol invariance not tracked if subclassed

[Hnasar]

Bug Report

Taking the Protocol with @property example from the docs: https://mypy.readthedocs.io/en/stable/protocols.html#invariance-of-protocol-attributes
The example shows that attributes must be read-only for subclass covariance to be inferred. This makes sense.

However, if a child class explicitly subclasses the Protocol then mypy doesn't report the error. This results in unsound code. This seems like a bug in typechecker behavior (and also docs which are confusing.)

To Reproduce

class Box(Protocol):
    content: object

def takes_box(box: Box) -> None:
    box.content = object()

@dataclasses.dataclass(slots=False)
class BadBox(Box):  # XXX: inheritence wrongly hides the error
    content: int

takes_box(BadBox(42))  # XXX should fail but doesn't!!

Full example here: https://mypy-play.net/?mypy=latest&python=3.11&gist=4d7e99ee4f933393234041a219e3f952

Expected Behavior

mypy to report that BadBox.content doesn't match Box, either in the declaration of content: int or in the function call to takes_box.

Actual Behavior

Mypy reports no errors, but at runtime, BadBox.content is no longer an int.

Notes

1. if Box.content: str, then a liskov error is reported for BadBox.content: int, but in the above example, because int is a subclass of object no error seems to be reported.

2. Perhaps ReadOnly will help here? https://peps.python.org/pep-0767/ I tried using Final but it's not allowed in Protocols unfortunately.

[brianschubert]

This sort of check is implemented by the error code mutable-override (disabled by default, not implied by --strict). If you enable mutable-override, you'll get the following error on the declaration of content: int:

main.py:15: error: Covariant override of a mutable attribute (base class "Box" defined the type as "object", expression has type "int")  [mutable-override]

playground: https://mypy-play.net/?mypy=latest&python=3.11&gist=4d7e99ee4f933393234041a219e3f952&enable-error-code=mutable-override