fix: object returned from api.configureWebpack should be merged

packages/@vue/cli-service/__tests__/Service.spec.js

@@ -140,6 +140,24 @@ test('api: configureWebpack', () => {
   expect(config.output.path).toBe('test-dist-2')
 })
 
+test('api: configureWebpack returning object', () => {
+  const service = createMockService([{
+    id: 'test',
+    apply: api => {
+      api.configureWebpack(config => {
+        return {
+          output: {
+            path: 'test-dist-3'
+          }
+        }
+      })
+    }
+  }])
+
+  const config = service.resolveWebpackConfig()
+  expect(config.output.path).toBe('test-dist-3')
+})
+
 test('api: configureDevServer', () => {
   const cb = () => {}
   const service = createMockService([{

packages/@vue/cli-service/lib/Service.js

@@ -143,7 +143,8 @@ module.exports = class Service {
     this.webpackRawConfigFns.forEach(fn => {
       if (typeof fn === 'function') {
         // function with optional return value
-        config = fn(config) || config
+        const res = fn(config)
+        if (res) config = merge(config, res)
       } else if (fn) {
         // merge literal values
         config = merge(config, fn)