Cannot use "ON CONFLICT" with postgres updatable view and partial index
After debugging through the PostgreSQL code, I'd say that there is a problem in the infer_arbiter_indexes function in src/backend/optimizer/util/plancat.c. It compares the index predicate to the WHERE condition in ON CONFLICT, but seems to ignore that they are on different objects.
I'm not sure if PostgreSQL is willing to consider that as a bug, but you might report it.