Currently the draft PEP specifies and the code supports the optional ability to add __slots__. This is the one place where @dataclass cannot just modify the given class and return it: because __slots__ must be specified at class creation time, it's too late by the time the dataclass decorator gets control. The current approach is to dynamically generate a new class while setting __slots__ in the new class and copying over other class attributes. The decorator then returns the new class.

The question is: do we even want to support setting __slots__? Is having __slots__ important enough to have this deviation from the "we just add a few dunder methods to your class" behavior?

I see three options:

1. Leave it as-is, with @dataclass(slots=True) returning a new class.
2. Completely remove support for setting __slots__.
3. Add a different decorator, say @add_slots, which takes a data class and creates a new class with __slots__ set.

I think we should either go with 2 or 3. I don't mind not supporting __slots__, but if we do want to support it, I think it's easier to explain with a separate decorator.

@add_slots
@dataclass
class C:
    x: int
    y: int

It would be an error to use @add_slots on a non-dataclass class.