[CLI] Exit non-zero with a clear message when next typegen fails (#94241)

The `typegen` command chained `nextTypegen(...).then(() =>
process.exit(0))` with no rejection handler. When `next.config` throws
during `loadConfig`, the rejection is orphaned by the synchronous
`program.parse` and then absorbed by Next's `unhandledRejection` filter,
so the process exits `0` having written no route types. A later `tsc`
then reports `Cannot find name 'PageProps'`/`'LayoutProps'` (and missing
`*.png` modules) with nothing tying it back to typegen. Add a `.catch`
that prints an actionable message about the missing route types and
`process.exit(1)`, so `next typegen && tsc` halts on failure. Includes
an e2e test whose fixture config throws and asserts a non-zero exit with
the diagnostic.

<!-- NEXT_JS_LLM_PR -->

packages/next/src/bin/next.ts

@@ -518,7 +518,20 @@ program
     // ensure process exits after typegen completes so open handles/connections
     // don't cause process to hang
     import('../cli/next-typegen.js').then((mod) =>
-      mod.nextTypegen(options, directory).then(() => process.exit(0))
+      mod
+        .nextTypegen(options, directory)
+        .then(() => process.exit(0))
+        .catch((err: unknown) => {
+          // Without this, a failed typegen (e.g. `next.config` throwing) is
+          // swallowed as an unhandled rejection that exits 0; surface it with a
+          // non-zero exit so `next typegen && tsc` halts instead of running
+          // against missing route types.
+          console.error(
+            '\n> Unexpected error while generating route types. Original error:\n'
+          )
+          console.error(err)
+          process.exit(1)
+        })
     )
   )
   .usage('[directory] [options]')

test/e2e/app-dir/typegen-error-diagnostic/app/layout.tsx

@@ -0,0 +1,8 @@
+import { ReactNode } from 'react'
+export default function Root({ children }: { children: ReactNode }) {
+  return (
+    <html>
+      <body>{children}</body>
+    </html>
+  )
+}

test/e2e/app-dir/typegen-error-diagnostic/app/page.tsx

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

test/e2e/app-dir/typegen-error-diagnostic/next.config.js

@@ -0,0 +1,25 @@
+// Faithful repro of the real-world trigger: `next typegen` runs with
+// NODE_ENV=production (see packages/next/src/bin/next.ts), which flips a
+// `warnOrError` helper to throw when a required env var is missing — exactly the
+// pattern apps like vercel-site use (e.g. CONTENTFUL_DELIVERY_TOKEN).
+//
+// In dev (e.g. during test harness setup) it only warns, so setup is unaffected;
+// during `next typegen` it throws, taking config loading — and therefore route
+// type generation — down with it.
+const isDev = process.env.NODE_ENV !== 'production'
+const warnOrError = isDev
+  ? console.warn
+  : (msg) => {
+      throw new Error(msg)
+    }
+
+if (!process.env.SOME_REQUIRED_TOKEN) {
+  warnOrError('SOME_REQUIRED_TOKEN is missing from env')
+}
+
+/**
+ * @type {import('next').NextConfig}
+ */
+const nextConfig = {}
+
+module.exports = nextConfig

test/e2e/app-dir/typegen-error-diagnostic/typegen-error-diagnostic.test.ts

@@ -0,0 +1,39 @@
+import { nextTestSetup } from 'e2e-utils'
+import { runNextCommand } from 'next-test-utils'
+
+describe('typegen error diagnostic', () => {
+  const { next, skipped } = nextTestSetup({
+    files: __dirname,
+    skipDeployment: true,
+    // We drive `next typegen` manually; no server needed.
+    skipStart: true,
+  })
+
+  if (skipped) {
+    return
+  }
+
+  it('fails loudly with an actionable message when route types cannot be generated', async () => {
+    // `next typegen` runs with NODE_ENV=production, which makes the fixture's
+    // next.config.js throw on the missing SOME_REQUIRED_TOKEN (the same shape as
+    // a real app's env assertion). typegen therefore cannot generate route types.
+    const { code, stdout, stderr } = await runNextCommand(
+      ['typegen', next.testDir],
+      { stderr: true, stdout: true }
+    )
+    const output = stdout + stderr
+
+    // Regression: `next typegen` used to swallow the failure (exit code 0) and
+    // write no route types, leaving the user with cryptic
+    // `Cannot find name 'PageProps'/'LayoutProps'` errors from a later `tsc` run.
+    // It must instead fail so `next typegen && tsc` chains halt.
+    expect(code).not.toBe(0)
+
+    // The message must connect the failure to route-type generation, so the user
+    // understands why PageProps/LayoutProps go missing.
+    expect(output).toMatch(/route types/i)
+
+    // ...and still surface the underlying cause.
+    expect(output).toContain('SOME_REQUIRED_TOKEN is missing from env')
+  })
+})