The following doesn't work in Python 3.7

from __future__ import annotations
import attr
from typing import ClassVar

@attr.dataclass
class A:
    x: ClassVar[int]

a = A()

Because ClassVar[int] will be 'ClassVar[int] in A.__annotations__ and so return str(annot).startswith("typing.ClassVar") returns False.

You can repro without 3.7 using this:

import attr
from typing import ClassVar

@attr.dataclass
class A:
    x: 'ClassVar[int]'

a = A()

Note: It's weird I'm not giving it a value but if I do then attrs will think it's a default value and the code won't error out.

Oh and the following works fine:

import attr
import typing

@attr.dataclass
class A:
    x: 'typing.ClassVar[int]'

a = A()