Loose types of app routes return value (#55849)

### What

#51394 introduced a pretty strict type of return value of route type
that causing failure with `next build`.
There're few ways of writing a app route, it could contain few return
values based on the usage:

* return a `Response` or promise of it
* return `NextResponse` of promise of it, since it's extended from
`Response`, same type
* use `redirect()` or `notFound(), since it returns `never`, and the
below code is not reached, the handler itself could still return void.
e.g. using `redirect` in a `GET` route

We loosed the type so `redirect()` can be still allowed without
specifying the return value there.
Related typescript issue:
microsoft/TypeScript#16608 (comment)

### How
* Re-enable the bail on types / build error in the app-routes tests
* Separate the tests, move runtime erroring ones to
`test/e2e/app-dir/app-routes-errors`
* Add new case to app-routes tests of mixed return value

Closes #55623 
Related #55604

packages/next/src/build/webpack/plugins/next-types-plugin/index.ts

@@ -107,11 +107,17 @@ if ('${method}' in entry) {
       '${method}'
     >
   >()
+  ${
+    ''
+    // Adding void to support never return type without explicit return:
+    // e.g. notFound() will interrupt the execution but the handler return type is inferred as void.
+    // x-ref: https://github.com/microsoft/TypeScript/issues/16608#issuecomment-309327984
+  }
   checkFields<
     Diff<
       {
         __tag__: '${method}',
-        __return_type__: Response | Promise<Response>
+        __return_type__: Response | void | never | Promise<Response | void | never>
       },
       {
         __tag__: '${method}',
@@ -428,7 +434,7 @@ declare module 'next/link' {
   import type { LinkProps as OriginalLinkProps } from 'next/dist/client/link.js'
   import type { AnchorHTMLAttributes, DetailedHTMLProps } from 'react'
   import type { UrlObject } from 'url'
-  
+
   type LinkRestProps = Omit<
     Omit<
       DetailedHTMLProps<

packages/next/src/trace/trace-uploader.ts

@@ -2,7 +2,6 @@ import findUp from 'next/dist/compiled/find-up'
 import fsPromise from 'fs/promises'
 import child_process from 'child_process'
 import assert from 'assert'
-// @ts-ignore
 import fetch from 'next/dist/compiled/node-fetch'
 import os from 'os'
 import { createInterface } from 'readline'

packages/next/types/misc.d.ts

@@ -43,6 +43,12 @@ declare module 'next/dist/compiled/@next/react-refresh-utils/dist/ReactRefreshWe
   export = m
 }
 
+declare module 'next/dist/compiled/node-fetch' {
+  import fetch from 'node-fetch'
+  export * from 'node-fetch'
+  export default fetch
+}
+
 declare module 'next/dist/compiled/node-html-parser' {
   export * from 'node-html-parser'
 }

test/development/app-dir/app-routes-error/app/lowercase/delete/route.js

@@ -0,0 +1 @@
+export { DELETE as delete } from '../../../hello'

test/development/app-dir/app-routes-error/app/lowercase/get/route.js

@@ -0,0 +1 @@
+export { GET as get } from '../../../hello'

test/development/app-dir/app-routes-error/app/lowercase/head/route.js

@@ -0,0 +1 @@
+export { HEAD as head } from '../../../hello'

test/development/app-dir/app-routes-error/app/lowercase/options/route.js

@@ -0,0 +1 @@
+export { OPTIONS as options } from '../../../hello'

test/development/app-dir/app-routes-error/app/lowercase/patch/route.js

@@ -0,0 +1 @@
+export { PATCH as patch } from '../../../hello'

test/development/app-dir/app-routes-error/app/lowercase/post/route.js

@@ -0,0 +1 @@
+export { POST as post } from '../../../hello'

test/development/app-dir/app-routes-error/app/lowercase/put/route.js

@@ -0,0 +1 @@
+export { PUT as put } from '../../../hello'

test/development/app-dir/app-routes-error/hello.js

@@ -0,0 +1,15 @@
+const helloHandler = async () => {
+  if (typeof WebSocket === 'undefined') {
+    throw new Error('missing WebSocket constructor!!')
+  }
+
+  return new Response('hello, world')
+}
+
+export const GET = helloHandler
+export const HEAD = helloHandler
+export const OPTIONS = helloHandler
+export const POST = helloHandler
+export const PUT = helloHandler
+export const DELETE = helloHandler
+export const PATCH = helloHandler

test/development/app-dir/app-routes-error/index.test.ts

@@ -0,0 +1,40 @@
+import { createNextDescribe } from 'e2e-utils'
+import { check } from 'next-test-utils'
+
+createNextDescribe(
+  'app-dir - app routes errors',
+  {
+    files: __dirname,
+  },
+  ({ next }) => {
+    describe('bad lowercase exports', () => {
+      it.each([
+        ['get'],
+        ['head'],
+        ['options'],
+        ['post'],
+        ['put'],
+        ['delete'],
+        ['patch'],
+      ])(
+        'should print an error when using lowercase %p in dev',
+        async (method: string) => {
+          await next.fetch('/lowercase/' + method)
+
+          await check(() => {
+            expect(next.cliOutput).toContain(
+              `Detected lowercase method '${method}' in`
+            )
+            expect(next.cliOutput).toContain(
+              `Export the uppercase '${method.toUpperCase()}' method name to fix this error.`
+            )
+            expect(next.cliOutput).toMatch(
+              /Detected lowercase method '.+' in '.+\/route\.js'\. Export the uppercase '.+' method name to fix this error\./
+            )
+            return 'yes'
+          }, 'yes')
+        }
+      )
+    })
+  }
+)

test/e2e/app-dir/app-routes/app-custom-routes.test.ts

@@ -589,45 +589,25 @@ createNextDescribe(
     })
 
     if (isNextDev) {
-      describe('lowercase exports', () => {
-        it.each([
-          ['get'],
-          ['head'],
-          ['options'],
-          ['post'],
-          ['put'],
-          ['delete'],
-          ['patch'],
-        ])(
-          'should print an error when using lowercase %p in dev',
-          async (method: string) => {
-            await next.fetch(basePath + '/lowercase/' + method)
-
-            await check(() => {
-              expect(next.cliOutput).toContain(
-                `Detected lowercase method '${method}' in`
-              )
-              expect(next.cliOutput).toContain(
-                `Export the uppercase '${method.toUpperCase()}' method name to fix this error.`
-              )
-              expect(next.cliOutput).toMatch(
-                /Detected lowercase method '.+' in '.+\/route\.ts'\. Export the uppercase '.+' method name to fix this error\./
-              )
-              return 'yes'
-            }, 'yes')
-          }
-        )
-      })
-
       describe('invalid exports', () => {
+        beforeAll(async () => {
+          await next.patchFile(
+            'app/default/route.ts',
+            `\
+          export { GET as default } from '../../handlers/hello'
+          `
+          )
+        })
+        afterAll(async () => {
+          await next.deleteFile('app/default/route.ts')
+        })
         it('should print an error when exporting a default handler in dev', async () => {
-          const res = await next.fetch(basePath + '/default')
-
-          // Ensure we get a 405 (Method Not Allowed) response when there is no
-          // exported handler for the GET method.
-          expect(res.status).toEqual(405)
+          await check(async () => {
+            const res = await next.fetch(basePath + '/default')
 
-          await check(() => {
+            // Ensure we get a 405 (Method Not Allowed) response when there is no
+            // exported handler for the GET method.
+            expect(res.status).toEqual(405)
             expect(next.cliOutput).toMatch(
               /Detected default export in '.+\/route\.ts'\. Export a named export for each HTTP method instead\./
             )

test/e2e/app-dir/app-routes/app/mixed-response/route.ts

@@ -0,0 +1,12 @@
+import { redirect } from 'next/navigation'
+import { NextResponse } from 'next/server'
+
+export async function GET() {
+  if (process.env.COND_1) {
+    return NextResponse.json({ a: '1' })
+  } else if (process.env.COND_2) {
+    redirect('/no-response')
+  } else {
+    return new Response('3')
+  }
+}

test/e2e/app-dir/app-routes/helpers.ts

@@ -60,10 +60,10 @@ type Cookies = {
 export function getRequestMeta(
   headersOrCookies:
     | Headers
-    | import('node-fetch').Headers
     | Cookies
     | ReadonlyHeaders
     | ReadonlyRequestCookies
+    | import('next/dist/compiled/node-fetch').Headers
 ): Record<string, any> {
   const headerOrCookie = headersOrCookies.get(KEY)
   if (!headerOrCookie) return {}

test/e2e/app-dir/app-routes/next.config.js

@@ -1,11 +1,7 @@
 /**
  * @type {import('next').NextConfig}
  */
-const config = {
-  typescript: {
-    ignoreBuildErrors: true,
-  },
-}
+const config = {}
 
 if (process.env.BASE_PATH) {
   config.basePath = process.env.BASE_PATH