fix event initialization on reused slot nodes (fix #3518)

Author: yyx990803

Diff for: src/core/vdom/helpers.js

@@ -92,10 +92,13 @@ export function updateListeners (
       if (Array.isArray(cur)) {
         add(event, (cur.invoker = arrInvoker(cur)), capture)
       } else {
-        fn = cur
-        cur = on[name] = {}
-        cur.fn = fn
-        add(event, (cur.invoker = fnInvoker(cur)), capture)
+        if (!cur.invoker) {
+          fn = cur
+          cur = on[name] = {}
+          cur.fn = fn
+          cur.invoker = fnInvoker(cur)
+        }
+        add(event, cur.invoker, capture)
       }
     } else if (Array.isArray(old)) {
       old.length = cur.length
@@ -126,6 +129,7 @@ function arrInvoker (arr: Array<Function>): Function {
 function fnInvoker (o: { fn: Function }): Function {
   return function (ev) {
     const single = arguments.length === 1
+    if (typeof o.fn !== 'function') debugger
     single ? o.fn(ev) : o.fn.apply(null, arguments)
   }
 }

Diff for: test/unit/features/component/component-slot.spec.js

@@ -481,4 +481,32 @@ describe('Component slot', () => {
     }).$mount()
     expect('Static <slot> found inside v-for').toHaveBeenWarned()
   })
+
+  // #3518
+  fit('events should not break when slot is toggled by v-if', done => {
+    const spy = jasmine.createSpy()
+    const vm = new Vue({
+      template: `<test><div class="click" @click="test">hi</div></test>`,
+      methods: {
+        test: spy
+      },
+      components: {
+        test: {
+          data: () => ({
+            toggle: true
+          }),
+          template: `<div v-if="toggle"><slot></slot></div>`
+        }
+      }
+    }).$mount()
+
+    expect(vm.$el.textContent).toBe('hi')
+    vm.$children[0].toggle = false
+    waitForUpdate(() => {
+      vm.$children[0].toggle = true
+    }).then(() => {
+      triggerEvent(vm.$el.querySelector('.click'), 'click')
+      expect(spy).toHaveBeenCalled()
+    }).then(done)
+  })
 })