fix(runtime-core): respect render function from mixins

fix #1630

packages/runtime-core/src/componentOptions.ts

@@ -4,7 +4,8 @@ import {
   SetupContext,
   ComponentInternalOptions,
   PublicAPIComponent,
-  Component
+  Component,
+  InternalRenderFunction
 } from './component'
 import {
   isFunction,
@@ -390,6 +391,7 @@ export function applyOptions(
     deactivated,
     beforeUnmount,
     unmounted,
+    render,
     renderTracked,
     renderTriggered,
     errorCaptured
@@ -398,7 +400,10 @@ export function applyOptions(
   const publicThis = instance.proxy!
   const ctx = instance.ctx
   const globalMixins = instance.appContext.mixins
-  // call it only during dev
+
+  if (asMixin && render && instance.render === NOOP) {
+    instance.render = render as InternalRenderFunction
+  }
 
   // applyOptions is called non-as-mixin once per instance
   if (!asMixin) {
@@ -406,6 +411,7 @@ export function applyOptions(
     // global mixins are applied first
     applyMixins(instance, globalMixins, deferredData, deferredWatch)
   }
+
   // extending a base component...
   if (extendsOptions) {
     applyOptions(instance, extendsOptions, deferredData, deferredWatch, true)