Equatable base dataclasses become inequatable when used as BaseModel fields

[silby]

Checks

• [ * ] I added a descriptive title to this issue
• [ * ] I have searched (google, github) for similar issues and couldn't find anything
• [ * ] I have read and followed the docs and still think this is a bug

Bug

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.9.0
            pydantic compiled: True
                 install path: /Users/silby/Documents/code/pydantic-wtf/venv/lib/python3.10/site-packages/pydantic
               python version: 3.10.0 (v3.10.0:b494f5935c, Oct  4 2021, 14:59:19) [Clang 12.0.5 (clang-1205.0.22.11)]
                     platform: macOS-11.5.2-arm64-arm-64bit
     optional deps. installed: ['typing-extensions']

When using a base dataclass created with eq=True, values of that class used as fields in a BaseModel inheritor lose their equatable property when wrapped by Pydantic. This seems to contradict the docs, which state "the generated pydantic dataclass will have the exact same configuration (order, frozen, ...) as the original one."

from dataclasses import dataclass
from pydantic import BaseModel

@dataclass(frozen=True, eq=True)
class A:
    id: int


class PairOfA(BaseModel):
    name: str
    left_a: A
    right_a: A


aa = A(1)
ab = A(1)

print(aa)
print(ab)

assert aa == ab

pair = PairOfA(name="same", left_a=aa, right_a=ab)

print(pair)

assert pair.left_a == pair.right_a

The second assertion above fails as written. When replacing dataclasses.dataclass with pydantic.dataclasses.dataclass it succeeds, which is our workaround for now, but it seems like this shouldn't be necessary.