dataclass.replace: allow transformed classes

[ikonst]

We already synthesize __dataclass_fields__ for all classes including @dataclass_transform'd ones, thus assume more than PEP-681 says. We might as well assume dataclasses.replace applies on all same classes. This way we risk false positive since it'll raise in runtime.

Fixes #15843.

[ikonst]

@AlexWaygood WDYT?

[AlexWaygood]

This makes sense to me, but I don't think it fixes the commented-out test in typeshed, which is trying to check that you can use is_dataclass in a typeguard-y way, i.e.:

from dataclasses import is_dataclass, replace

def foo(x: object):
    if is_dataclass(x) and not isinstance(x, type):
        y = replace(x)

I think mypy is still going to complain about ^that kind of usage, but it's idiomatic code according to the dataclasses documentation.

For reference, the signature that we have for is_dataclass in typeshed is:

@overload
def is_dataclass(obj: DataclassInstance) -> Literal[True]: ...
@overload
def is_dataclass(obj: type) -> TypeGuard[type[DataclassInstance]]: ...
@overload
def is_dataclass(obj: object) -> TypeGuard[DataclassInstance | type[DataclassInstance]]: ...

And the signature we have for dataclasses.replace is:

def replace(__obj: _DataclassT, **changes: Any) -> _DataclassT: ...

Where DataclassInstance and _DataclassT are:

class DataclassInstance(Protocol):
    __dataclass_fields__: ClassVar[dict[str, dataclasses.Field[Any]]]

_DataclassT = TypeVar("_DataclassT", bound=DataclassInstance)

[ikonst]

Thanks for explaining so thoroughly.

[ikonst]

👉 #15962

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[ikonst]

@AlexWaygood shall we go ahead with this?