Currently ClassVar isn't implemented. I have opened python/typeshed#888 and submitted python/typeshed#889 (I hope it's enough). Once it's get merged mypy should implement this.

I would like to tackle this. I have started implementing this with ClassVar definition from PR I had submitted to typeshed and so far mypy is able to treat ClassVar[T] as T, which is expected. Of course assignment semantics are main purpose of ClassVar and I plan to implement them.

By the way, should mypy treat objects of this type as immutable, that is report errors on operations like calling append on List? AFAIK mypy doesn't have any concept of mutability.