This issue is waiting a patch? https://clojure.atlassian.net/browse/CLJ-2426 If yes, where should "via-metadata" should be? Is between "interface" and "subclasses" a good place?

(defn find-protocol-impl [protocol x]
  (if (instance? (:on-interface protocol) x)
    x
    (or
      (when (:extend-via-metadata protocol)
        ...find-via-metadata...)
      (let [c (class x)
            impl #(get (:impls protocol) %)]
        ...find-via-class-hierarchy....))))

we haven't decided what to do about this yet