fix(traverse): Prevent traversing Vue instances

Related: vuejs/vuex#1587

Author: SergioCrisostomo

src/core/observer/traverse.js

@@ -3,6 +3,8 @@
 import { _Set as Set, isObject } from '../util/index'
 import type { SimpleSet } from '../util/index'
 import VNode from '../vdom/vnode'
+import Vue from '../instance/index'
+
 
 const seenObjects = new Set()
 
@@ -19,7 +21,7 @@ export function traverse (val: any) {
 function _traverse (val: any, seen: SimpleSet) {
   let i, keys
   const isA = Array.isArray(val)
-  if ((!isA && !isObject(val)) || Object.isFrozen(val) || val instanceof VNode) {
+  if ((!isA && !isObject(val)) || Object.isFrozen(val) || val instanceof VNode || val instanceof Vue) {
     return
   }
   if (val.__ob__) {

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

@@ -106,6 +106,29 @@ describe('Options watch', () => {
     }).then(done)
   })
 
+  it('with option: deep and with reference to a Vue instance', done => {
+    const vueInstance = new Vue();
+    const vm = new Vue({
+      data: { a: { b: vueInstance }},
+      watch: {
+        a: {
+          handler: spy,
+          deep: true
+        }
+      }
+    })
+    const oldA = vm.a
+    expect(spy).not.toHaveBeenCalled()
+    vm.a.b = 2
+    expect(spy).not.toHaveBeenCalled()
+    waitForUpdate(() => {
+      expect(spy).toHaveBeenCalledWith(vm.a, vm.a)
+      vm.a = { b: 3 }
+    }).then(() => {
+      expect(spy).toHaveBeenCalledWith(vm.a, oldA)
+    }).then(done)
+  })
+
   it('correctly merges multiple extends', done => {
     const spy2 = jasmine.createSpy('A')
     const spy3 = jasmine.createSpy('B')