fix(slots): should render fallback content when slot content contains no valid nodes (#2485)

fix #2347, fix #2461

packages/runtime-core/__tests__/helpers/renderSlot.spec.ts

@@ -5,7 +5,8 @@ import {
   createVNode,
   openBlock,
   createBlock,
-  Fragment
+  Fragment,
+  createCommentVNode
 } from '../../src'
 import { PatchFlags } from '@vue/shared/src'
 
@@ -47,4 +48,31 @@ describe('renderSlot', () => {
     const templateRendered = renderSlot({ default: slot }, 'default')
     expect(templateRendered.dynamicChildren!.length).toBe(1)
   })
+
+  // #2347 #2461
+  describe('only render valid slot content', () => {
+    it('should ignore slots that are all comments', () => {
+      let fallback
+      const vnode = renderSlot(
+        { default: () => [createCommentVNode('foo')] },
+        'default',
+        undefined,
+        () => [(fallback = h('fallback'))]
+      )
+      expect(vnode.children).toEqual([fallback])
+      expect(vnode.patchFlag).toBe(PatchFlags.BAIL)
+    })
+
+    it('should ignore invalid slot content generated by nested slot', () => {
+      let fallback
+      const vnode = renderSlot(
+        { default: () => [renderSlot({}, 'foo')] },
+        'default',
+        undefined,
+        () => [(fallback = h('fallback'))]
+      )
+      expect(vnode.children).toEqual([fallback])
+      expect(vnode.patchFlag).toBe(PatchFlags.BAIL)
+    })
+  })
 })