Fix apply of IFn for more than 20 arguments. (#167)

Co-authored-by: David Frese <david.frese@active-group.de>

Author: swannodette

src/main/clojure/cljs/core.cljc

@@ -1499,15 +1499,20 @@
 (core/defn- add-ifn-methods [type type-sym [f & meths :as form]]
   (core/let [meths    (map #(adapt-ifn-params type %) meths)
              this-sym (with-meta 'self__ {:tag type})
-             argsym   (gensym "args")]
+             argsym   (gensym "args")
+             max-ifn-arity 20]
     (concat
       [`(set! ~(extend-prefix type-sym 'call) ~(with-meta `(fn ~@meths) (meta form)))
        `(set! ~(extend-prefix type-sym 'apply)
           ~(with-meta
              `(fn ~[this-sym argsym]
                 (this-as ~this-sym
                   (.apply (.-call ~this-sym) ~this-sym
-                    (.concat (array ~this-sym) (cljs.core/aclone ~argsym)))))
+                    (.concat (array ~this-sym)
+                      (if (> (.-length ~argsym) ~max-ifn-arity)
+                        (doto (.slice ~argsym 0 ~max-ifn-arity)
+                          (.push (.slice ~argsym ~max-ifn-arity)))
+                        ~argsym)))))
              (meta form)))]
       (ifn-invoke-methods type type-sym form))))
 

src/test/cljs/cljs/apply_test.cljs

@@ -42,14 +42,11 @@
   (is (= '(1 2 3 4)) (apply meta-f 1 2 3 4 []))
   (is (= '(1 2 3 4 5)) (apply meta-f 1 2 3 4 5 []))
   (is (= (range 1 8)) (apply meta-f 1 2 3 4 5 [6 7]))
-  ;; Currently: 20 is not seqable :(
-  #_(is (= (range 21) (apply meta-f (range 21)))
+  (is (= (range 21) (apply meta-f (range 21)))
         "Should properly call the last IFn arity with 20 args with last being a seq")
-  ;; Currently: Tries to call arity 21. Fault at .apply of the deftype proto
-  ;; Though, it could probably also be caught right by apply
-  #_(is (= (range 22) (apply meta-f (range 22)))
+  (is (= (range 22) (apply meta-f (range 22)))
         "Should properly call the last IFn arity with 20 args with last being a seq")
-  #_(is (= (range 22) (.apply meta-f nil (to-array (range 22))))
+  (is (= (range 22) (.apply meta-f nil (to-array (range 22))))
         ".apply should also handle >20 arguments"))
 
 (deftest multi-arity-test