fix: reconcile the overridden prototype of component and _Vue mocks and mixins

Author: kuitos

Diff for: packages/create-instance/create-instance.js

@@ -1,5 +1,6 @@
 // @flow
 
+import Vue from 'vue'
 import { createSlotVNodes } from './create-slot-vnodes'
 import addMocks from './add-mocks'
 import { addEventLogger } from './log-events'
@@ -109,10 +110,35 @@ export default function createInstance (
     component.options._base = _Vue
   }
 
+  function getRootVueProto (obj) {
+    while (obj) {
+      if (Object.getPrototypeOf(obj) === Vue.prototype) {
+        return obj
+      }
+
+      obj = Object.getPrototypeOf(obj)
+    }
+  }
+
+  function getExtendedComponent (component, instanceOptions) {
+    // extend _Vue to merge the mixins on _Vue
+    const extendedComponent = component.extend(_Vue).extend(instanceOptions)
+
+    // cache subclass constructor
+    component.options._Ctor[extendedComponent.cid] = extendedComponent
+
+    // to keep the possible overridden prototype and _Vue mocks on prototype,
+    // we need change the proto chains manually
+    // @see https://github.com/vuejs/vue-test-utils/pull/856
+    const root = getRootVueProto(extendedComponent.prototype)
+    Object.setPrototypeOf(root, _Vue.prototype)
+
+    return extendedComponent
+  }
+
   // extend component from _Vue to add properties and mixins
-  // extend does not work correctly for sub class components in Vue < 2.2
-  const Constructor = typeof component === 'function' && vueVersion < 2.3
-    ? component.extend(instanceOptions)
+  const Constructor = typeof component === 'function'
+    ? getExtendedComponent(component, instanceOptions)
     : _Vue.extend(component).extend(instanceOptions)
 
   // Keep reference to component mount was called with

Diff for: test/specs/mount.spec.js

@@ -156,7 +156,7 @@ describeRunIf(process.env.TEST_ENV !== 'node', 'mount', () => {
       expect(stub).not.called
     })
 
-  it.skip('overrides component prototype', () => {
+  it('overrides component prototype', () => {
     const mountSpy = sinon.spy()
     const destroySpy = sinon.spy()
     const Component = Vue.extend({})