Yury, thanks for your patch. I'll review it soon.

Please note that postponed annotations only reveal a problem that we already had if anybody used a string forward reference:

>>> from dataclasses import dataclass
>>> from typing import get_type_hints
>>> class C:
...   pass
...
>>> @dataclass
... class D:
...   c: C
...
>>> @dataclass
... class E:
...   c: "C"
...
>>> get_type_hints(C.__init__)
{}
>>> get_type_hints(D.__init__)
{'c': <class '__main__.C'>, 'return': <class 'NoneType'>}
>>> get_type_hints(E.__init__)
Traceback (most recent call last):
...
NameError: name 'C' is not defined