A simple example is documented here: https://github.com/samuelcolvin/pydantic/issues/2233 but it doesn't look like it was actually filed as a bug against CPython... Copy paste of the minimal reproducing example:

from dataclasses import asdict, dataclass

@dataclass(eq=True, frozen=True)
class A:
  a: str

@dataclass(eq=True, frozen=True)
class B:
  b: dict[A, str]

asdict(B({A("x"):"y"}))


The offending code are these lines in dataclasses.py:

    elif isinstance(obj, dict):
        return type(obj)((_asdict_inner(k, dict_factory),
                          _asdict_inner(v, dict_factory))

The _asdict_inner(k, dict_factory) [correctly] turns dataclasses into dicts... which are not hashable. I assume there is a good reason the _asdict_inner function is called on the key, but its failure in this case looks like a (rather annoying!) error.