is_dataclass doesn't promise DataclassInstance

[NeilGirdhar]

from dataclasses import is_dataclass

def f(X: type) -> None:
    if is_dataclass(X):
        reveal_type(X)  # information: Type of "X" is "type"

Should this be the typeshed type type[DataclassInstance]? As far as I can tell, this is

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

Related typing-sig post.

[erictraut]

I think pyright is doing the right thing here.

Here's the overloaded definition of is_dataclass from typeshed:

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

You are passing an argument of type type which is the equivalent of type[Any]. This matches the first overload, which does not incorporate a TypeGuard.

I recommend that the overload ordering in typeshed be swapped or the first overload be deleted.