Make sure the global not found route doesn't conflict with existing /not-found route

packages/next/src/build/entries.ts

@@ -92,6 +92,7 @@ export function createPagesMapping({
       let pageKey = getPageFromPath(pagePath, pageExtensions)
       if (isAppRoute) {
         pageKey = pageKey.replace(/%5F/g, '_')
+        pageKey = pageKey.replace(/^\/not-found$/g, '/_not-found')
       }
 
       if (pageKey in result) {

packages/next/src/build/index.ts

@@ -678,7 +678,7 @@ export default async function build(
 
       const conflictingPublicFiles: string[] = []
       const hasPages404 = mappedPages['/404']?.startsWith(PAGES_DIR_ALIAS)
-      const hasApp404 = !!mappedAppPages?.['/not-found']
+      const hasApp404 = !!mappedAppPages?.['/_not-found']
       const hasCustomErrorPage =
         mappedPages['/_error'].startsWith(PAGES_DIR_ALIAS)
 
@@ -2493,7 +2493,7 @@ export default async function build(
 
             routes.forEach((route) => {
               if (isDynamicRoute(page) && route === page) return
-              if (route === '/not-found') return
+              if (route === '/_not-found') return
 
               let revalidate = exportConfig.initialPageRevalidationMap[route]
 
@@ -2703,7 +2703,7 @@ export default async function build(
                   distDir,
                   'server',
                   'app',
-                  'not-found.html'
+                  '_not-found.html'
                 )
                 const updatedRelativeDest = path
                   .join('pages', '404.html')

packages/next/src/export/worker.ts

@@ -462,7 +462,7 @@ export default async function exportPage({
           try {
             curRenderOpts.params ||= {}
 
-            const isNotFoundPage = page === '/not-found'
+            const isNotFoundPage = page === '/_not-found'
             const result = await renderToHTMLOrFlight(
               req as any,
               res as any,

packages/next/src/server/base-server.ts

@@ -2248,7 +2248,7 @@ export default abstract class Server<ServerOptions extends Options = Options> {
         if (this.hasAppDir) {
           // Use the not-found entry in app directory
           result = await this.findPageComponents({
-            pathname: '/not-found',
+            pathname: '/_not-found',
             query,
             params: {},
             isAppPath: true,

test/e2e/app-dir/not-found/app/not-found/page.js

@@ -0,0 +1,3 @@
+export default function Page() {
+  return <h1>I'm still a valid page</h1>
+}

test/e2e/app-dir/not-found/not-found.test.ts

@@ -13,6 +13,11 @@ createNextDescribe(
         expect(html).toContain('This Is The Not Found Page')
       })
 
+      it('should allow to have a valid /not-found route', async () => {
+        const html = await next.render('/not-found')
+        expect(html).toContain("I'm still a valid page")
+      })
+
       if (!isNextDev) {
         it('should create the 404 mapping and copy the file to pages', async () => {
           const html = await next.readFile('.next/server/pages/404.html')