relax orphan rule for (define-instance (A:A :x => B:B :x) ...)

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The orphan rule seems overly restrictive for what I imagine is a somewhat common case of defining a new class, and implementing instances implied by that class. Specifically, if A:A (package A with class A) is defined and we wish to define an instance of B:B, like so:

(define-instance (A:A :t => B:B :t) ...)

then we need to define this in the package B. This is inconvenient, because it means that we would need to put A:A very early in our project, divorcing it from additional logic/library functionality.

I propose we just add this as a special case to the orphan rule. I don't think we need to generalize it (e.g., (A1:A1 :t) (A2:A2 :t) => B:B :t) or anything like that), but it might be worth it.

Any downsides to doing this?

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CC @macrologist

[macrologist]

One minor minus that comes to mind is that this would lead to a degree of arbitrariness in the decision about where to put multiply-constrained typeclass instances.

(define-instance ((A:A :t) (B:B :t) => C:C :t) ...)

where, by convention, do we put this? A.lisp or B.lisp ?

Others?

[stylewarning]

where, by convention, do we put this? A.lisp or B.lisp ?

(FWIW, I would think it's usually the case that one of these is the load-bearing one.)