dataclasses is no doubt too lenient. But it's just trying to accept valid strings that look like ClassVar. Way back when this was initially implemented, we decided that calling get_type_hints would be too expensive for every dataclass, and would also neccesitate importing typing, which we didn't want to require.

I think someone needs to do an analysis of how expensive it would be for dataclasses to import typing and to call get_type_hints. Perhaps we'd make a different decision today.