CLJS-1518: Case macro expansion evaluates expression twice

Store the value of the case expression in a local.

Author: swannodette

src/main/clojure/cljs/core.cljc

@@ -2229,7 +2229,8 @@
                          vec
                          (mapv #(if (seq? %) (vec %) [%])))
                  thens (vec (vals pairs))]
-        `(let [~esym (if (keyword? ~e) (.-fqn ~e) nil)]
+        `(let [~esym ~e
+               ~esym (if (keyword? ~esym) (.-fqn ~esym) nil)]
            (case* ~esym ~tests ~thens ~default)))
 
       ;; equality

src/test/cljs/cljs/core_test.cljs

@@ -1195,6 +1195,14 @@
     (is (= x [1 2]))
     (is (realized? x))))
 
+(deftest test-1518
+  (testing "Test evaluate expression once - keyword tests"
+    (let [m {:a :b
+             :b :c}
+          x (atom :a)]
+      (case (swap! x m) :a 0 :default)
+      (is (= :b @x)))))
+
 (comment
   ;; ObjMap
   ;; (let [ks (map (partial str "foo") (range 500))