fix: show error when "next start" is used with "output" config

docs/api-reference/cli.md

@@ -82,7 +82,7 @@ Or using the `PORT` environment variable:
 PORT=4000 npx next dev
 ```
 
-> Note: `PORT` can not be set in `.env` as booting up the HTTP server happens before any other code is initialized.
+> Note: `PORT` cannot be set in `.env` as booting up the HTTP server happens before any other code is initialized.
 
 You can also set the hostname to be different from the default of `0.0.0.0`, this can be useful for making the application available for other devices on the network. The default hostname can be changed with `-H`, like so:
 
@@ -106,7 +106,9 @@ Or using the `PORT` environment variable:
 PORT=4000 npx next start
 ```
 
-> Note: `PORT` can not be set in `.env` as booting up the HTTP server happens before any other code is initialized.
+> Note: `PORT` cannot be set in `.env` as booting up the HTTP server happens before any other code is initialized.
+
+> Note: `next start` cannot be used with `output: 'standalone'` or `output: 'export'`.
 
 ### Keep Alive Timeout
 

packages/next/src/server/next.ts

@@ -164,6 +164,17 @@ export class NextServer {
   private async getServer() {
     if (!this.serverPromise) {
       this.serverPromise = this.loadConfig().then(async (conf) => {
+        if (!this.options.dev) {
+          if (conf.output === 'standalone') {
+            log.warn(
+              `"next start" does not work with "output: standalone" configuration. Use "node .next/standalone/server.js" instead.`
+            )
+          } else if (conf.output === 'export') {
+            throw new Error(
+              `"next start" does not work with "output: export" configuration. Use "npx serve@latest out" instead.`
+            )
+          }
+        }
         if (conf.experimental.appDir) {
           process.env.NEXT_PREBUNDLED_REACT = '1'
           overrideBuiltInReactPackages()

test/integration/app-dir-export/test/index.test.ts

@@ -201,7 +201,7 @@ async function runTests({
   }
 }
 
-describe('app dir with output export', () => {
+describe('app dir with output export (next dev / next build)', () => {
   it.each([
     { isDev: true, trailingSlash: false },
     { isDev: true, trailingSlash: true },

test/integration/app-dir-export/test/start.test.ts

@@ -0,0 +1,60 @@
+/* eslint-env jest */
+
+import { join } from 'path'
+import fs from 'fs-extra'
+import {
+  check,
+  File,
+  findPort,
+  killApp,
+  nextBuild,
+  nextStart,
+} from 'next-test-utils'
+
+const appDir = join(__dirname, '..')
+const distDir = join(appDir, '.next')
+const exportDir = join(appDir, 'out')
+const nextConfig = new File(join(appDir, 'next.config.js'))
+let app
+
+describe('app dir with output export (next start)', () => {
+  afterEach(async () => {
+    await killApp(app)
+    nextConfig.restore()
+    await fs.remove(distDir)
+    await fs.remove(exportDir)
+  })
+
+  it('should error during next start with output export', async () => {
+    const { code } = await nextBuild(appDir)
+    expect(code).toBe(0)
+    const port = await findPort()
+    let stderr = ''
+    app = await nextStart(appDir, port, {
+      onStderr(msg: string) {
+        stderr += msg || ''
+      },
+    })
+    await check(() => stderr, /error/i)
+    expect(stderr).toContain(
+      '"next start" does not work with "output: export" configuration. Use "npx serve@latest out" instead.'
+    )
+  })
+
+  it('should warn during next start with output standalone', async () => {
+    nextConfig.replace(`output: 'export'`, `output: 'standalone'`)
+    const { code } = await nextBuild(appDir)
+    expect(code).toBe(0)
+    const port = await findPort()
+    let stderr = ''
+    app = await nextStart(appDir, port, {
+      onStderr(msg: string) {
+        stderr += msg || ''
+      },
+    })
+    await check(() => stderr, /warn/i)
+    expect(stderr).toContain(
+      `warn  - "next start" does not work with "output: standalone" configuration. Use "node .next/standalone/server.js" instead.`
+    )
+  })
+})