fix: should listen for config file changes when specifying the app directory

packages/next/bin/next.ts

@@ -144,18 +144,3 @@ commands[command]()
       process.exit(0)
     }
   })
-
-if (command === 'dev') {
-  const { CONFIG_FILES } = require('../shared/lib/constants')
-  const { watchFile } = require('fs')
-
-  for (const CONFIG_FILE of CONFIG_FILES) {
-    watchFile(`${process.cwd()}/${CONFIG_FILE}`, (cur: any, prev: any) => {
-      if (cur.size > 0 || prev.size > 0) {
-        console.log(
-          `\n> Found a change in ${CONFIG_FILE}. Restart the server to see the changes in effect.`
-        )
-      }
-    })
-  }
-}

packages/next/cli/next-dev.ts

@@ -1,13 +1,15 @@
 #!/usr/bin/env node
 import arg from 'next/dist/compiled/arg/index.js'
-import { existsSync } from 'fs'
+import { existsSync, watchFile } from 'fs'
 import { startServer } from '../server/lib/start-server'
 import { printAndExit } from '../server/lib/utils'
 import * as Log from '../build/output/log'
 import { startedDevelopmentServer } from '../build/output'
 import { cliCommand } from '../bin/next'
 import isError from '../lib/is-error'
 import { getProjectDir } from '../lib/get-project-dir'
+import { CONFIG_FILES } from '../shared/lib/constants'
+import path from 'path'
 
 const nextDev: cliCommand = (argv) => {
   const validArgs: arg.Spec = {
@@ -128,6 +130,16 @@ const nextDev: cliCommand = (argv) => {
       }
       process.nextTick(() => process.exit(1))
     })
+
+  for (const CONFIG_FILE of CONFIG_FILES) {
+    watchFile(path.join(dir, CONFIG_FILE), (cur: any, prev: any) => {
+      if (cur.size > 0 || prev.size > 0) {
+        console.log(
+          `\n> Found a change in ${CONFIG_FILE}. Restart the server to see the changes in effect.`
+        )
+      }
+    })
+  }
 }
 
 export { nextDev }

test/development/watch-config-file/index.test.ts

@@ -0,0 +1,49 @@
+import { createNext } from 'e2e-utils'
+import { check } from 'next-test-utils'
+import { NextInstance } from 'test/lib/next-modes/base'
+
+describe('watch-config-file', () => {
+  let next: NextInstance
+
+  beforeAll(async () => {
+    next = await createNext({
+      files: {
+        'pages/index.js': `
+          export default function Page() { 
+            return <p>hello world</p>
+          } 
+        `,
+        'next.config.js': `
+        const nextConfig = {
+          reactStrictMode: true,
+        }
+
+        module.exports = nextConfig        
+        `,
+      },
+      dependencies: {},
+      skipStart: true,
+    })
+  })
+  afterAll(() => next.destroy())
+
+  it('should output config file change', async () => {
+    // next dev test-dir
+    await next.start(true)
+
+    await next.patchFile(
+      'next.config.js',
+      `
+      /** changed **/  
+      const nextConfig = {
+        reactStrictMode: true,
+      }
+      module.exports = nextConfig`
+    )
+
+    await check(
+      () => next.cliOutput,
+      /Found a change in next.config.js. Restart the server to see the changes in effect./
+    )
+  })
+})

test/lib/next-modes/base.ts

@@ -238,7 +238,7 @@ export class NextInstance {
     return {}
   }
   public async setup(): Promise<void> {}
-  public async start(): Promise<void> {}
+  public async start(useDirArg: boolean = false): Promise<void> {}
   public async stop(): Promise<void> {
     this.isStopping = true
     if (this.childProcess) {

test/lib/next-modes/next-dev.ts

@@ -16,19 +16,21 @@ export class NextDevInstance extends NextInstance {
     return this._cliOutput || ''
   }
 
-  public async start() {
+  public async start(useDirArg: boolean = false) {
     if (this.childProcess) {
       throw new Error('next already started')
     }
-    let startArgs = ['yarn', 'next']
+    let startArgs = ['yarn', 'next', useDirArg && this.testDir].filter(
+      Boolean
+    ) as string[]
 
     if (this.startCommand) {
       startArgs = this.startCommand.split(' ')
     }
 
     await new Promise<void>((resolve) => {
       this.childProcess = spawn(startArgs[0], startArgs.slice(1), {
-        cwd: this.testDir,
+        cwd: useDirArg ? process.cwd() : this.testDir,
         stdio: ['ignore', 'pipe', 'pipe'],
         shell: false,
         env: {