> the compiler has no sane answer once both impls are in the graph

but the user could provide an answer.

Julia allows something called "piracy" which is spiritually similar to the orphan rule. and while it is strongly discouraged, the compiler allows it. and when "ambiguities" arise (here called "coherence") it becomes a social problem, not technical, to resolve. and usually package authors are pretty willing to cooperate to resolve it!