CLJS-1673: Can't spec instrument a multimethod

fix multimethods case. add tests for multimethod case and multi-arity fn case.

Author: dnolen

src/main/cljs/cljs/spec.cljs

@@ -291,7 +291,7 @@
                                   (str "Call to " (pr-str v) " did not conform to spec:\n" (with-out-str (explain-out ed)))
                                   ed)))
                        conformed)))]
-    (doto
+    (cond->
       (c/fn
         [& args]
         (if *instrument-enabled*
@@ -305,7 +305,7 @@
                   (conform! v :fn (:fn specs) {:args cargs :ret cret} args))
                 ret)))
           (apply f args)))
-      (gobj/extend f))))
+      (not (instance? MultiFn f)) (doto (gobj/extend f)))))
 
 (defn- macroexpand-check
   [v args]

src/test/cljs/cljs/spec_test.cljs

@@ -1,6 +1,6 @@
 (ns cljs.spec-test
   (:require [cljs.spec :as s]
-            [cljs.test :as test :refer-macros [deftest is]]))
+            [cljs.test :as test :refer-macros [deftest is run-tests]]))
 
 (s/def ::even? (s/and number? even?))
 (s/def ::odd? (s/and number? odd?))
@@ -16,6 +16,33 @@
   (let [xs [42 11 13 15 {:a 1 :b 2 :c 3} 1 2 3 42 43 44 11]]
     (is (= xs (s/unform s2 (s/conform s2 xs))))))
 
+(defn adder
+  ([a] a)
+  ([a b] (+ a b)))
+
+(s/fdef adder
+  :args (s/cat :a integer? :b (s/? integer?))
+  :ret integer?)
+
+(s/instrument #'adder)
+
+(deftest test-multi-arity-instrument
+  (is (= 1 (adder 1)))
+  (is (= 3 (adder 1 2)))
+  (is (thrown? js/Error (adder "foo"))))
+
+(defmulti testmm :type)
+(defmethod testmm :default [_])
+(defmethod testmm :good [_] "good")
+
+(s/fdef testmm :args (s/cat :m map?) :ret string?)
+
+(s/instrument #'testmm)
+
+(deftest test-multifn-instrument
+  (is (= "good" (testmm {:type :good})))
+  (is (thrown? js/Error (testmm "foo"))))
+
 (comment
 
   (s/conform s2 [42 11 13 15 {:a 1 :b 2 :c 3} 1 2 3 42 43 44 11])