`dataclasses.asdict()` should accept dataclasses

[grievejia]

Describe the Bug

Minimal repro:

import dataclasses

@dataclasses.dataclass
class Foo:
    x: int  = 0

def test(foo: Foo) -> None:
    dataclasses.asdict(foo)

Expected: no error
Actual: Argument Foo is not assignable to parameter obj with type DataclassInstance in function dataclasses.asdict

Sandbox Link

https://pyrefly.org/sandbox/?code=JYWwDg9gTgLgBAEwIYyQYwDZIM7YKbYBQhAAsqpjvtgHTnpa6GW5wBiEEAXIXH3AA8ucYADt4cALxwADMQR4AZnBgEYACkWdhHCAEo4AWgB8cAHIRReHv0QoGVAjRwJgaDVv1A

(Only applicable for extension issues) IDE Information

No response

[rchen152]

Oh DataclassInstance is a protocol that checks for the presence of __dataclass_fields__: https://github.com/python/typeshed/blob/6007267f68ed94a49421cff17b03e23a0177ad8c/stdlib/_typeshed/__init__.pyi#L352. So we probably just need to synthesize that field.