CLJS-1663: Calling instrumented multi-arity function causes exception

when instrumenting we set! the var to the spec verifying fn but this meant
that arity references for correct dispatching would be missing

link the arities from the original function object to the spec verifying
fn via goog.object/extend

Author: dnolen

src/main/cljs/cljs/spec.cljs

@@ -10,7 +10,8 @@
   (:refer-clojure :exclude [+ * and or cat def keys])
   (:require-macros [cljs.core :as c]
                    [cljs.spec :as s])
-  (:require [cljs.core :as c]
+  (:require [goog.object :as gobj]
+            [cljs.core :as c]
             [clojure.walk :as walk]
             [cljs.spec.impl.gen :as gen]
             [clojure.string :as str]))
@@ -280,19 +281,21 @@
                                   (str "Call to " v " did not conform to spec:\n" (with-out-str (explain-out ed)))
                                   ed)))
                        conformed)))]
-    (c/fn
-      [& args]
-      (if *instrument-enabled*
-        (s/with-instrument-disabled
-          (let [specs (fn-specs v)]
-            (let [cargs (when (:args specs) (conform! v :args (:args specs) args args))
-                  ret (binding [*instrument-enabled* true]
-                        (apply f args))
-                  cret (when (:ret specs) (conform! v :ret (:ret specs) ret args))]
-              (when (c/and (:args specs) (:ret specs) (:fn specs))
-                (conform! v :fn (:fn specs) {:args cargs :ret cret} args))
-              ret)))
-        (apply f args)))))
+    (doto
+      (c/fn
+        [& args]
+        (if *instrument-enabled*
+          (s/with-instrument-disabled
+            (let [specs (fn-specs v)]
+              (let [cargs (when (:args specs) (conform! v :args (:args specs) args args))
+                    ret (binding [*instrument-enabled* true]
+                          (apply f args))
+                    cret (when (:ret specs) (conform! v :ret (:ret specs) ret args))]
+                (when (c/and (:args specs) (:ret specs) (:fn specs))
+                  (conform! v :fn (:fn specs) {:args cargs :ret cret} args))
+                ret)))
+          (apply f args)))
+      (gobj/extend f))))
 
 ;(defn- macroexpand-check
 ;  [v args]