Update extendRoutes modules API

lib/core/module.js

@@ -74,7 +74,25 @@ export default class ModuleContainer extends Tapable {
   }
 
   extendRoutes (fn) {
-    this.options.router.extendRoutes = chainFn(this.options.router.extendRoutes, fn)
+    const extendRoutesFromConfig = this.options.router.extendRoutes
+    this.options.router.extendRoutes = function (routes, resolve) {
+      /* istanbul ignore if */
+      if (!(fn instanceof Function)) {
+        return
+      }
+      if (typeof extendRoutesFromConfig !== 'function') {
+        return fn.call(this, routes, resolve)
+      }
+      let extendedRoutes = extendRoutesFromConfig.call(this, routes, resolve)
+      if (extendedRoutes === undefined) {
+        extendedRoutes = routes
+      }
+      let moduleExtendedRoutes = fn.call(this, extendedRoutes, resolve)
+      if (moduleExtendedRoutes === undefined) {
+        return extendedRoutes
+      }
+      return moduleExtendedRoutes
+    }
   }
 
   requireModule (moduleOpts) {

test/fixtures/module/modules/basic/index.js

@@ -20,6 +20,7 @@ module.exports = function basicModule (options, resolve) {
   // Extend routes
   this.extendRoutes((routes, resolve) => {
     // Do nothing!
+    return routes
   })
 
   // Require same module twice