See #92052

I thought about how we could implement this for a bit -- and it looks like we would need to add an extra class attribute that would act as a flag, say __dataclass_decorator_called__, and then check whether it's been set or not in the wrap inner function. If it has been set, we raise a TypeError.

I guess the only caveat is if the class definition already contains an attribute with the same name as the flag, in which case, this implementation would erroneously raise an exception even if the @dataclass decorator was used just once.

@rohitsanj: All names that begin and end with __ (e.g. the proposed flag you give) are reserved for use by Python and subject to breakage with no warning. We're allowed to blithely destroy any and all code using __*__ names in any way that does not conform to the existing set of special methods, so using such a flag is not a problem in that sense.

Thanks for the clarification, @MojoVampire

I already have this coded up, using __dataclass_fields__ as the indicator. I'm holding off on it until the discussion in #92052 is resolved.

See PR #92518.