CLJS-3283 Support -contains-key? protocol check in contains?

Author: wilkerlucio

src/main/cljs/cljs/core.cljs

@@ -2360,8 +2360,14 @@ reduces them without incurring seq initialization"
   range of indexes. 'contains?' operates constant or logarithmic time;
   it will not perform a linear search for a value.  See also 'some'."
   [coll v]
-  (if (identical? (get coll v lookup-sentinel) lookup-sentinel)
+  (cond
+    (implements? IAssociative coll)
+    (-contains-key? coll v)
+
+    (identical? (get coll v lookup-sentinel) lookup-sentinel)
     false
+
+    :else
     true))
 
 (defn find

src/test/cljs/cljs/core_test.cljs

@@ -84,7 +84,14 @@
     (is (not (contains? (to-array [5 6 7]) 3)))
     (is (not (contains? nil 42)))
     (is (contains? "f" 0))
-    (is (not (contains? "f" 55)))))
+    (is (not (contains? "f" 55))))
+
+  (testing "Testing contains? with IAssociative protocol"
+    (let [ds (reify
+               IAssociative
+               (-contains-key? [_ k] (= k :valid)))]
+     (is (contains? ds :valid))
+     (is (not (contains? ds :invalid))))))
 
 (deftest test-run!
   (testing "Testing run!"