I said it over in the other thread, but do folks think that forbidding macros from synthesizing declarations but allowing them to fill in arbitrary definitions is a reasonable compromise? This would mean that @data would not be able to generate an apply or withX methods, but it could still fill in equals and hashCode. I think the withX methods could be done lens style with .with(_.x)(value) if there is a trait Data with def with[T](field: Self => T)(value: T): Self. Okay, Self doesn’t exist in Scala 3 (right?), but that’s a separate issue 