Add error if app detected but config is missing

packages/next/lib/find-pages-dir.ts

@@ -29,30 +29,27 @@ export function findPagesDir(
   appDir: string | undefined
 } {
   const pagesDir = findDir(dir, 'pages') || undefined
-  let appDir: undefined | string
+  const appDir = findDir(dir, 'app') || undefined
 
-  if (isAppDirEnabled) {
-    appDir = findDir(dir, 'app') || undefined
-  }
-  const hasAppDir =
-    !!appDir && fs.existsSync(appDir) && fs.statSync(appDir).isDirectory()
-
-  if (hasAppDir && appDir == null && pagesDir == null) {
+  if (isAppDirEnabled && appDir == null && pagesDir == null) {
     throw new Error(
       "> Couldn't find any `pages` or `app` directory. Please create one under the project root"
     )
   }
 
-  if (!isAppDirEnabled) {
-    if (pagesDir == null) {
+  if (!isAppDirEnabled && pagesDir == null) {
+    if (appDir != null) {
       throw new Error(
-        "> Couldn't find a `pages` directory. Please create one under the project root"
+        '> The `app` dir is experimental. Please add `{experimental:{appDir: true}}` to your `next.config.js` to enable it'
       )
     }
+    throw new Error(
+      "> Couldn't find a `pages` directory. Please create one under the project root"
+    )
   }
 
   return {
     pagesDir,
-    appDir,
+    appDir: isAppDirEnabled ? appDir : undefined,
   }
 }

test/integration/appdir-missing-config/app/page.js

@@ -0,0 +1,3 @@
+export default function Page() {
+  return <p>hello from app</p>
+}

test/integration/appdir-missing-config/test/index.test.js

@@ -0,0 +1,61 @@
+/* eslint-env jest */
+
+import path from 'path'
+import {
+  killApp,
+  findPort,
+  launchApp,
+  nextBuild,
+  waitFor,
+} from 'next-test-utils'
+
+let app
+let appPort
+const appDir = path.join(__dirname, '..')
+let output = ''
+const err =
+  /Error: > The `app` dir is experimental. Please add `{experimental:{appDir: true}}` to your `next.config.js` to enable it/
+
+function runTests() {
+  it('should print error for conflicting app/page', async () => {
+    expect(output).toMatch(err)
+  })
+}
+
+describe('Conflict between app file and page file', () => {
+  describe('next dev', () => {
+    beforeAll(async () => {
+      output = ''
+      appPort = await findPort()
+      app = await launchApp(appDir, appPort, {
+        onStdout(msg) {
+          output += msg || ''
+        },
+        onStderr(msg) {
+          output += msg || ''
+        },
+      })
+      await waitFor(200)
+    })
+    afterAll(() => {
+      if (app) killApp(app)
+    })
+    runTests()
+  })
+
+  describe('next build', () => {
+    beforeAll(async () => {
+      output = ''
+      const app = await nextBuild(appDir, [], {
+        stdout: true,
+        stderr: true,
+        env: { NEXT_SKIP_APP_REACT_INSTALL: '1' },
+      })
+      output = app.stdout + app.stderr
+    })
+    afterAll(() => {
+      if (app) killApp(app)
+    })
+    runTests()
+  })
+})