fix(slot): add a function to return the slot fallback content

src/compiler/codegen/index.js

@@ -546,7 +546,7 @@ export function genComment (comment: ASTText): string {
 function genSlot (el: ASTElement, state: CodegenState): string {
   const slotName = el.slotName || '"default"'
   const children = genChildren(el, state)
-  let res = `_t(${slotName}${children ? `,${children}` : ''}`
+  let res = `_t(${slotName}${children ? `,function(){return ${children}}` : ''}`
   const attrs = el.attrs || el.dynamicAttrs
     ? genProps((el.attrs || []).concat(el.dynamicAttrs || []).map(attr => ({
         // slot props are camelized

src/core/instance/render-helpers/render-slot.js

@@ -7,7 +7,7 @@ import { extend, warn, isObject } from 'core/util/index'
  */
 export function renderSlot (
   name: string,
-  fallback: ?Array<VNode>,
+  fallbackRender: ?() => Array<VNode>,
   props: ?Object,
   bindObject: ?Object
 ): ?Array<VNode> {
@@ -24,9 +24,9 @@ export function renderSlot (
       }
       props = extend(extend({}, bindObject), props)
     }
-    nodes = scopedSlotFn(props) || fallback
+    nodes = scopedSlotFn(props) || (fallbackRender && fallbackRender())
   } else {
-    nodes = this.$slots[name] || fallback
+    nodes = this.$slots[name] || (fallbackRender && fallbackRender())
   }
 
   const target = props && props.slot

test/unit/features/component/component-slot.spec.js

@@ -109,6 +109,26 @@ describe('Component slot', () => {
     expect(child.$el.children[1].textContent).toBe('slot b')
   })
 
+  it('fallback content should not be evaluated when the parent is providing it', () => {
+    const test = jasmine.createSpy('test')
+    const vm = new Vue({
+      template: '<test>slot default</test>',
+      components: {
+        test: {
+          template: '<div><slot>{{test()}}</slot></div>',
+          methods: {
+            test () {
+              test()
+              return 'test'
+            }
+          }
+        }
+      }
+    }).$mount()
+    expect(vm.$el.textContent).toBe('slot default')
+    expect(test).not.toHaveBeenCalled()
+  })
+
   it('selector matching multiple elements', () => {
     mount({
       childTemplate: '<div><slot name="t"></slot></div>',

test/unit/modules/compiler/codegen.spec.js

@@ -196,7 +196,7 @@ describe('codegen', () => {
   it('generate slot fallback content', () => {
     assertCodegen(
       '<div><slot><div>hi</div></slot></div>',
-      `with(this){return _c('div',[_t("default",[_c('div',[_v("hi")])])],2)}`
+      `with(this){return _c('div',[_t("default",function(){return [_c('div',[_v("hi")])]})],2)}`
     )
   })