Ensure _.mergeWith overwrites primitives with source object clones. [closes #1880]

lodash.js

@@ -3057,7 +3057,7 @@
     function baseMergeDeep(object, source, key, srcIndex, mergeFunc, customizer, stack) {
       var objValue = object[key],
           srcValue = source[key],
-          stacked = stack.get(srcValue) || stack.get(objValue);
+          stacked = stack.get(srcValue);
 
       if (stacked) {
         assignMergeValue(object, key, stacked);
@@ -3076,6 +3076,7 @@
             newValue = copyArray(objValue);
           }
           else {
+            isCommon = false;
             newValue = baseClone(srcValue);
           }
         }
@@ -3084,6 +3085,7 @@
             newValue = toPlainObject(objValue);
           }
           else if (!isObject(objValue) || (srcIndex && isFunction(objValue))) {
+            isCommon = false;
             newValue = baseClone(srcValue);
           }
           else {

test/test.js

@@ -13528,6 +13528,16 @@
 
       assert.deepEqual(actual, { 'a': { 'b': [0, 1, 2] } });
     });
+
+    QUnit.test('should overwrite primitives with source object clones', function(assert) {
+      assert.expect(1);
+
+      var actual = _.mergeWith({ 'a': 0 }, { 'a': { 'b': ['c'] } }, function(a, b) {
+        return lodashStable.isArray(a) ? a.concat(b) : undefined;
+      });
+
+      assert.deepEqual(actual, { 'a': { 'b': ['c'] } });
+    });
   }());
 
   /*--------------------------------------------------------------------------*/