CLJS-2943: Update merge-with to use key / val

Author: mfikes

src/main/cljs/cljs/core.cljs

@@ -9018,7 +9018,7 @@ reduces them without incurring seq initialization"
   [f & maps]
   (when (some identity maps)
     (let [merge-entry (fn [m e]
-                        (let [k (first e) v (second e)]
+                        (let [k (key e) v (val e)]
                           (if (contains? m k)
                             (assoc m k (f (get m k) v))
                             (assoc m k v))))

src/test/cljs/cljs/core_test.cljs

@@ -1625,3 +1625,15 @@
   (is (false? ((comp not empty?) "")))
   (is (thrown? js/Error ((not empty?) "foo")))
   (is (thrown? js/Error ((not empty?) ""))))
+
+(deftest test-cljs-2943
+  (let [m1 {:a 2, :b 3, :c 5}
+        m2 {:a 7, :b 11, :d 13, :e 17}
+        m3 {:a 19, :d 23, :f 29}
+        m4 {:a 28, :b 14, :c 5, :d 36, :e 17, :f 29}
+        sorted (fn [m] (into (sorted-map) m))]
+    (is (= m4 (merge-with + m1 m2 m3)))
+    (is (= m4 (merge-with + (sorted m1) m2 m3)))
+    (is (= m4 (merge-with + (sorted m1) (sorted m2) m3)))
+    (is (= m4 (merge-with + m1 (sorted m2) m3)))
+    (is (= m4 (merge-with + m1 (sorted m2) (sorted m3))))))