fix(ssr): respect render function from extends/mixins in ssr (#3006)

fix #3004

packages/server-renderer/__tests__/render.spec.ts

@@ -99,6 +99,46 @@ function testRender(type: string, render: typeof renderToString) {
         ).toBe(`<div>hello</div>`)
       })
 
+      test('components using defineComponent with extends option', async () => {
+        expect(
+          await render(
+            createApp(
+              defineComponent({
+                extends: {
+                  data() {
+                    return { msg: 'hello' }
+                  },
+                  render(this: any) {
+                    return h('div', this.msg)
+                  }
+                }
+              })
+            )
+          )
+        ).toBe(`<div>hello</div>`)
+      })
+
+      test('components using defineComponent with mixins option', async () => {
+        expect(
+          await render(
+            createApp(
+              defineComponent({
+                mixins: [
+                  {
+                    data() {
+                      return { msg: 'hello' }
+                    },
+                    render(this: any) {
+                      return h('div', this.msg)
+                    }
+                  }
+                ]
+              })
+            )
+          )
+        ).toBe(`<div>hello</div>`)
+      })
+
       test('optimized components', async () => {
         expect(
           await render(

packages/server-renderer/src/render.ts

@@ -22,7 +22,8 @@ import {
   isString,
   isVoidTag,
   ShapeFlags,
-  isArray
+  isArray,
+  NOOP
 } from '@vue/shared'
 import { ssrRenderAttrs } from './helpers/ssrRenderAttrs'
 import { ssrCompile } from './helpers/ssrCompile'
@@ -118,7 +119,7 @@ function renderComponentSubTree(
     )
   } else {
     if (
-      !instance.render &&
+      (!instance.render || instance.render === NOOP) &&
       !instance.ssrRender &&
       !comp.ssrRender &&
       isString(comp.template)
@@ -155,7 +156,7 @@ function renderComponentSubTree(
         instance.ctx
       )
       setCurrentRenderingInstance(null)
-    } else if (instance.render) {
+    } else if (instance.render && instance.render !== NOOP) {
       renderVNode(
         push,
         (instance.subTree = renderComponentRoot(instance)),