CLJS-3457: :lite-mode data structures should satisfy identical? if unchanged by conj

src/main/cljs/cljs/core.cljs

@@ -12603,10 +12603,14 @@ reduces them without incurring seq initialization"
     (let [k (if-not (keyword? k) k (keyword->obj-map-key k))]
       (if (string? k)
         (if-not (nil? (scan-array 1 k strkeys))
-          (let [new-strobj (obj-clone strobj strkeys)]
-            (gobject/set new-strobj k v)
-            (ObjMap. meta strkeys new-strobj nil))             ;overwrite
-          (let [new-strobj (obj-clone strobj strkeys)          ; append
+          (if (identical? v (gobject/get strobj k))
+            coll
+            ; overwrite
+            (let [new-strobj (obj-clone strobj strkeys)]
+              (gobject/set new-strobj k v)
+              (ObjMap. meta strkeys new-strobj nil))) 
+          ; append
+          (let [new-strobj (obj-clone strobj strkeys) 
                 new-keys (aclone strkeys)]
             (gobject/set new-strobj k v)
             (.push new-keys k)
@@ -12812,10 +12816,12 @@ reduces them without incurring seq initialization"
               i (scan-array-equiv 2 k new-bucket)]
           (aset new-hashobj h new-bucket)
           (if (some? i)
-            (do
-              ; found key, replace
-              (aset new-bucket (inc i) v)
-              (HashMap. meta count new-hashobj nil))
+            (if (identical? v (aget new-bucket (inc i)))
+              coll
+              (do
+                ; found key, replace
+                (aset new-bucket (inc i) v)
+                (HashMap. meta count new-hashobj nil)))
             (do
               ; did not find key, append
               (.push new-bucket k v)
@@ -12954,7 +12960,10 @@ reduces them without incurring seq initialization"
 
   ICollection
   (-conj [coll o]
-    (Set. meta (assoc hash-map o o) nil))
+    (let [new-hash-map (assoc hash-map o o)]
+      (if (identical? new-hash-map hash-map)
+        coll
+        (Set. meta new-hash-map nil))))
 
   IEmptyableCollection
   (-empty [coll] (with-meta (. Set -EMPTY) meta))

src/test/cljs/cljs/lite_collections_test.cljs

@@ -28,6 +28,14 @@
   (= 1 (aget (clj->js (obj-map :x 1)) "x"))
   (= 1 (aget (clj->js {:x 1}) "x")))
 
+(deftest test-unchanged-identical?
+  (let [m (obj-map :foo 1)]
+    (identical? m (assoc m :foo 1)))
+  (let [m (simple-hash-map :foo 1)]
+    (identical? m (assoc m :foo 1)))
+  (let [s (simple-set [:foo])]
+    (identical? s (conj s :foo))))
+
 (comment
 
   (require '[cljs.lite-collections-test] :reload)