Extend not-found.js to catch all unmatched routes

packages/next/src/build/index.ts

@@ -523,7 +523,17 @@ export default async function build(
         appPaths = await nextBuildSpan
           .traceChild('collect-app-paths')
           .traceAsyncFn(() =>
-            recursiveReadDir(appDir, validFileMatcher.isAppRouterPage)
+            recursiveReadDir(appDir, (absolutePath) => {
+              if (validFileMatcher.isAppRouterPage(absolutePath)) {
+                return true
+              }
+              // For now we only collect the root /not-found page in the app
+              // directory as the 404 fallback.
+              if (validFileMatcher.isRootNotFound(absolutePath)) {
+                return true
+              }
+              return false
+            })
           )
       }
 
@@ -653,6 +663,7 @@ export default async function build(
 
       const conflictingPublicFiles: string[] = []
       const hasPages404 = mappedPages['/404']?.startsWith(PAGES_DIR_ALIAS)
+      const hasApp404 = !!mappedAppPages?.['/not-found']
       const hasCustomErrorPage =
         mappedPages['/_error'].startsWith(PAGES_DIR_ALIAS)
 
@@ -2159,7 +2170,8 @@ export default async function build(
       // Since custom _app.js can wrap the 404 page we have to opt-out of static optimization if it has getInitialProps
       // Only export the static 404 when there is no /_error present
       const useStatic404 =
-        !customAppGetInitialProps && (!hasNonStaticErrorPage || hasPages404)
+        !customAppGetInitialProps &&
+        (!hasNonStaticErrorPage || hasPages404 || hasApp404)
 
       if (invalidPages.size > 0) {
         const err = new Error(
@@ -2463,6 +2475,7 @@ export default async function build(
 
             routes.forEach((route) => {
               if (isDynamicRoute(page) && route === page) return
+              if (route === '/not-found') return
 
               let revalidate = exportConfig.initialPageRevalidationMap[route]
 
@@ -2664,9 +2677,36 @@ export default async function build(
               })
           }
 
-          // Only move /404 to /404 when there is no custom 404 as in that case we don't know about the 404 page
-          if (!hasPages404 && useStatic404) {
-            await moveExportedPage('/_error', '/404', '/404', false, 'html')
+          async function moveExportedAppNotFoundTo404() {
+            return staticGenerationSpan
+              .traceChild('move-exported-app-not-found-')
+              .traceAsyncFn(async () => {
+                const orig = path.join(
+                  distDir,
+                  'server',
+                  'app',
+                  'not-found.html'
+                )
+                const updatedRelativeDest = path
+                  .join('pages', '404.html')
+                  .replace(/\\/g, '/')
+                await promises.copyFile(
+                  orig,
+                  path.join(distDir, 'server', updatedRelativeDest)
+                )
+                pagesManifest['/404'] = updatedRelativeDest
+              })
+          }
+
+          // If there's /not-found inside app, we prefer it over the pages 404
+          if (hasApp404 && useStatic404) {
+            // await moveExportedPage('/_error', '/404', '/404', false, 'html')
+            await moveExportedAppNotFoundTo404()
+          } else {
+            // Only move /404 to /404 when there is no custom 404 as in that case we don't know about the 404 page
+            if (!hasPages404 && useStatic404) {
+              await moveExportedPage('/_error', '/404', '/404', false, 'html')
+            }
           }
 
           if (useDefaultStatic500) {

packages/next/src/build/webpack/loaders/next-app-loader.ts

@@ -219,9 +219,13 @@ async function createTreeCodeFromPath(
         })
       )
 
+      const definedFilePaths = filePaths.filter(
+        ([, filePath]) => filePath !== undefined
+      )
+
       if (!rootLayout) {
-        const layoutPath = filePaths.find(
-          ([type, filePath]) => type === 'layout' && !!filePath
+        const layoutPath = definedFilePaths.find(
+          ([type]) => type === 'layout'
         )?.[1]
         rootLayout = layoutPath
 
@@ -232,9 +236,6 @@ async function createTreeCodeFromPath(
         }
       }
 
-      const definedFilePaths = filePaths.filter(
-        ([, filePath]) => filePath !== undefined
-      )
       props[parallelKey] = `[
         '${
           Array.isArray(parallelSegment) ? parallelSegment[0] : parallelSegment

packages/next/src/export/worker.ts

@@ -465,10 +465,11 @@ export default async function exportPage({
           try {
             curRenderOpts.params ||= {}
 
+            const isNotFoundPage = page === '/not-found'
             const result = await renderToHTMLOrFlight(
               req as any,
               res as any,
-              page,
+              isNotFoundPage ? '/404' : page,
               query,
               curRenderOpts as any
             )

packages/next/src/server/app-render/index.tsx

@@ -679,6 +679,7 @@ export async function renderToHTMLOrFlight(
               rootLayoutIncluded: rootLayoutIncludedAtThisLevelOrAbove,
               injectedCSS: injectedCSSWithCurrentLayout,
               injectedFontPreloadTags: injectedFontPreloadTagsWithCurrentLayout,
+              asNotFound,
             })
 
             const childProp: ChildProp = {
@@ -739,8 +740,24 @@ export async function renderToHTMLOrFlight(
 
       const isClientComponent = isClientReference(layoutOrPageMod)
 
+      // If it's a not found route, and we don't have any matched parallel
+      // routes, we try to render the not found component if it exists.
+      let notFoundComponent = {}
+      if (asNotFound && !parallelRouteMap.length && NotFound) {
+        notFoundComponent = {
+          children: (
+            <>
+              <meta name="robots" content="noindex" />
+              {notFoundStyles}
+              <NotFound />
+            </>
+          ),
+        }
+      }
+
       const props = {
         ...parallelRouteComponents,
+        ...notFoundComponent,
         // TODO-APP: params and query have to be blocked parallel route names. Might have to add a reserved name list.
         // Params are always the current params that apply to the layout
         // If you have a `/dashboard/[team]/layout.js` it will provide `team` as a param but not anything further down.
@@ -847,6 +864,7 @@ export async function renderToHTMLOrFlight(
         injectedCSS,
         injectedFontPreloadTags,
         rootLayoutIncluded,
+        asNotFound,
       }: {
         createSegmentPath: CreateSegmentPath
         loaderTreeToFilter: LoaderTree
@@ -858,6 +876,7 @@ export async function renderToHTMLOrFlight(
         injectedCSS: Set<string>
         injectedFontPreloadTags: Set<string>
         rootLayoutIncluded: boolean
+        asNotFound?: boolean
       }): Promise<FlightDataPath> => {
         const [segment, parallelRoutes, components] = loaderTreeToFilter
 
@@ -931,6 +950,7 @@ export async function renderToHTMLOrFlight(
                       injectedFontPreloadTags,
                       // This is intentionally not "rootLayoutIncludedAtThisLevelOrAbove" as createComponentTree starts at the current level and does a check for "rootLayoutAtThisLevel" too.
                       rootLayoutIncluded: rootLayoutIncluded,
+                      asNotFound,
                     }
                   )
 
@@ -988,6 +1008,7 @@ export async function renderToHTMLOrFlight(
             injectedCSS: injectedCSSWithCurrentLayout,
             injectedFontPreloadTags: injectedFontPreloadTagsWithCurrentLayout,
             rootLayoutIncluded: rootLayoutIncludedAtThisLevelOrAbove,
+            asNotFound,
           })
 
           if (typeof path[path.length - 1] !== 'string') {
@@ -1025,6 +1046,7 @@ export async function renderToHTMLOrFlight(
             injectedCSS: new Set(),
             injectedFontPreloadTags: new Set(),
             rootLayoutIncluded: false,
+            asNotFound: pathname === '/404',
           })
         ).slice(1),
       ]
@@ -1410,7 +1432,11 @@ export async function renderToHTMLOrFlight(
     }
     // End of action request handling.
 
-    const renderResult = new RenderResult(await bodyResult({}))
+    const renderResult = new RenderResult(
+      await bodyResult({
+        asNotFound: pathname === '/404',
+      })
+    )
 
     if (staticGenerationStore.pendingRevalidates) {
       await Promise.all(staticGenerationStore.pendingRevalidates)

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

@@ -2181,16 +2181,30 @@ export default abstract class Server<ServerOptions extends Options = Options> {
       let using404Page = false
 
       // use static 404 page if available and is 404 response
-      if (is404 && (await this.hasPage('/404'))) {
-        result = await this.findPageComponents({
-          pathname: '/404',
-          query,
-          params: {},
-          isAppPath: false,
-          // Ensuring can't be done here because you never "match" a 404 route.
-          shouldEnsure: true,
-        })
-        using404Page = result !== null
+      if (is404) {
+        if (this.hasAppDir) {
+          // Use the not-found entry in app directory
+          result = await this.findPageComponents({
+            pathname: '/not-found',
+            query,
+            params: {},
+            isAppPath: true,
+            shouldEnsure: true,
+          })
+          using404Page = result !== null
+        }
+
+        if (!result && (await this.hasPage('/404'))) {
+          result = await this.findPageComponents({
+            pathname: '/404',
+            query,
+            params: {},
+            isAppPath: false,
+            // Ensuring can't be done here because you never "match" a 404 route.
+            shouldEnsure: true,
+          })
+          using404Page = result !== null
+        }
       }
       let statusPage = `/${res.statusCode}`
 

packages/next/src/server/lib/app-dir-module.ts

@@ -1,4 +1,4 @@
-import { ComponentsType } from '../../build/webpack/loaders/next-app-loader'
+import type { ComponentsType } from '../../build/webpack/loaders/next-app-loader'
 
 /**
  * LoaderTree is generated in next-app-loader.

packages/next/src/server/lib/find-page-file.ts

@@ -91,6 +91,9 @@ export function createValidFileMatcher(
       pageExtensions
     )}$`
   )
+  const leafOnlyNotFoundFileRegex = new RegExp(
+    `^not-found\\.${getExtensionRegexString(pageExtensions)}$`
+  )
   /** TODO-METADATA: support other metadata routes
    *  regex for:
    *
@@ -125,9 +128,21 @@ export function createValidFileMatcher(
     return validExtensionFileRegex.test(filePath) || isMetadataFile(filePath)
   }
 
+  function isRootNotFound(filePath: string) {
+    if (!appDirPath) {
+      return false
+    }
+    if (!filePath.startsWith(appDirPath + sep)) {
+      return false
+    }
+    const rest = filePath.slice(appDirPath.length + 1)
+    return leafOnlyNotFoundFileRegex.test(rest)
+  }
+
   return {
     isPageFile,
     isAppRouterPage,
     isMetadataFile,
+    isRootNotFound,
   }
 }

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

@@ -0,0 +1,10 @@
+export default function Layout({ children }) {
+  return (
+    <html>
+      <head>
+        <title>Hello World</title>
+      </head>
+      <body>{children}</body>
+    </html>
+  )
+}

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

@@ -0,0 +1,3 @@
+export default function Page() {
+  return <h1>This Is The Not Found Page</h1>
+}

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

@@ -0,0 +1,3 @@
+export default function Page() {
+  return <h1>My page</h1>
+}

test/e2e/app-dir/not-found/next.config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  experimental: {
+    appDir: true,
+  },
+}

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

@@ -0,0 +1,27 @@
+import { createNextDescribe } from 'e2e-utils'
+
+createNextDescribe(
+  'app dir - not-found',
+  {
+    files: __dirname,
+    skipDeployment: true,
+  },
+  ({ next, isNextDev }) => {
+    describe('root not-found page', () => {
+      it('should use the not-found page for non-matching routes', async () => {
+        const html = await next.render('/random-content')
+        expect(html).toContain('This Is The Not Found 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')
+          expect(html).toContain('This Is The Not Found Page')
+          expect(
+            await next.readFile('.next/server/pages-manifest.json')
+          ).toContain('"pages/404.html"')
+        })
+      }
+    })
+  }
+)