Added check in merge strat for watches if child is already an array (fix #5652) (#5653)

* Added check for if child is already an array

If the child is already an array it does not need to be wrapped again.
Fixing #5652

* Added unit test for watch merge strat

* Moved test to own describe

* Added test for merging multiple extends

src/core/util/options.js

@@ -172,7 +172,7 @@ strats.watch = function (parentVal: ?Object, childVal: ?Object): ?Object {
     }
     ret[key] = parent
       ? parent.concat(child)
-      : [child]
+      : Array.isArray(child) ? child : [child]
   }
   return ret
 }

test/unit/features/options/extends.spec.js

@@ -47,3 +47,44 @@ describe('Options extends', () => {
     expect(vm.c).toBe(3)
   })
 })
+
+describe('Options extends with Object.prototype.watch', () => {
+  beforeAll(function () {
+    if (!Object.prototype.watch) {
+      // eslint-disable-next-line no-extend-native
+      Object.prototype.watch = {
+        remove: true
+      }
+    }
+  })
+  afterAll(function () {
+    if (Object.prototype.watch && Object.prototype.watch.remove) {
+      delete Object.prototype.watch
+    }
+  })
+  it('should work with global mixins', done => {
+    Vue.use({
+      install: function () {
+        Vue.mixin({})
+      }
+    })
+    const spy = jasmine.createSpy('watch')
+    const A = Vue.extend({
+      data: function () {
+        return { a: 1 }
+      },
+      watch: {
+        a: spy
+      },
+      created: function () {
+        this.a = 2
+      }
+    })
+    new Vue({
+      extends: A
+    })
+    waitForUpdate(() => {
+      expect(spy).toHaveBeenCalledWith(2, 1)
+    }).then(done)
+  })
+})

test/unit/features/options/watch.spec.js

@@ -104,4 +104,43 @@ describe('Options watch', () => {
       expect(spy).toHaveBeenCalledWith(vm.a, oldA)
     }).then(done)
   })
+
+  it('correctly merges multiple extends', done => {
+    var spy2 = jasmine.createSpy('A')
+    var spy3 = jasmine.createSpy('B')
+    var A = Vue.extend({
+      data: function () {
+        return {
+          a: 0,
+          b: 0
+        }
+      },
+      watch: {
+        b: spy
+      }
+    })
+
+    var B = Vue.extend({
+      extends: A,
+      watch: {
+        a: spy2
+      }
+    })
+
+    var C = Vue.extend({
+      extends: B,
+      watch: {
+        a: spy3
+      }
+    })
+
+    var vm = new C()
+    vm.a = 1
+
+    waitForUpdate(() => {
+      expect(spy).not.toHaveBeenCalled()
+      expect(spy2).toHaveBeenCalledWith(1, 0)
+      expect(spy3).toHaveBeenCalledWith(1, 0)
+    }).then(done)
+  })
 })