The Hashtable Problem: A Litmus Test for External Impl Proposals

jrose:

Therefore "the Hash implementation is part of the type that depends on Hash" is not a solution for the HashMap in std.

Without further additions, at least. It's afaict okay as long as the impl identity constraint is enforced for any function calls where both implementing and depending type appear in the signature bounds-linked (even indirectly, so basically any case where those types are linked by relevant type parameter bounds).