False positive error on subclass overriding a ClassVar #4715
Description
This seems to be a bug.
With the following code:
from typing import ClassVar
class Base:
name: ClassVar[str]
class Sub(Base):
name = "Sub"
mypy complains that :7: error: Cannot override class variable (previously declared on base class "Base") with instance variable.
But the assignment on the subclass is clearly at the class level. Assigning a value to a class attribute that has already been declared as one should not result in an error. I'd expect this error only if the assignment took place e.g. inside __init__.
Explicitly annotating the subclass assignment (name: ClassVar[str] = "Sub") gets rid of the error, but that repeated annotation should not be necessary.
Reproduced on mypy 0.580-dev-9374acf2d808905d392b2ec8ea52bfce03f9ecad under Python 3.6.4, with no flags or options.
I realize that inability to distinguish instance vs class attributes is a known mypy limitation (#1097), but in general it just results in false negatives. This seems worth a distinct issue since it appears that mypy is attempting to make the distinction, but doing so incorrectly, resulting in a false positive.