from dataclasses import dataclass
from typing import Dict

from typing_extensions import Protocol

@dataclass
class A:
    pass

class IsDataclass(Protocol):
    __dataclass_fields__: Dict

def dataclass_only(x: IsDataclass):
    pass

dataclass_only(A())

When running mypy on this code, I'd expect no warning, but I get

test.py:16: error: Argument 1 to "dataclass_only" has incompatible type "A"; expected "IsDataclass".

While checking for __dataclass_fields__ might not look like the best thing to go for, it's what dataclasses.is_dataclass checks for as well, so this seems like the most reliable condition right now.

I'd assume it happens because the dataclass-wrapper adds this attribute, and mypy only checks the class definition itself. Can this be fixed?