Ah, so you mean a caller would potentially not know which groups would be used for cascaded validation since conversions defined on a sub-type might apply. That's indeed right.

So to be super-correct we would have to enforce the same rules for conversions on return values as for parameter constraints and conversions on parameters (may not be specified on methods overriding/implementing a super-type method). Given that we have this restriction in place for parameter conversions anyway, it would probably make things not much more worse when we did the same for conversions on return values.