Ensure 404 with SSG is rendered correctly with notFound (#18205)

This ensures a custom `/404` page with `getStaticProps` works correctly when leveraging the new `unstable_notFound` support in `getStaticProps`

Closes: https://github.com/vercel/next.js/issues/18196
x-ref: https://github.com/vercel/next.js/pull/17755

packages/next/next-server/server/next-server.ts

@@ -1183,8 +1183,10 @@ export default class Server {
     { components, query }: FindComponentsResult,
     opts: RenderOptsPartial
   ): Promise<string | null> {
+    const is404Page = pathname === '/404'
+
     // we need to ensure the status code if /404 is visited directly
-    if (pathname === '/404') {
+    if (is404Page) {
       res.statusCode = 404
     }
 
@@ -1255,13 +1257,19 @@ export default class Server {
       urlPathname = stripNextDataPath(urlPathname)
     }
 
-    const ssgCacheKey =
+    let ssgCacheKey =
       isPreviewMode || !isSSG
         ? undefined // Preview mode bypasses the cache
         : `${locale ? `/${locale}` : ''}${resolvedUrlPathname}${
             query.amp ? '.amp' : ''
           }`
 
+    if (is404Page && isSSG) {
+      ssgCacheKey = `${locale ? `/${locale}` : ''}${pathname}${
+        query.amp ? '.amp' : ''
+      }`
+    }
+
     // Complete the response with cached data if its present
     const cachedData = ssgCacheKey
       ? await this.incrementalCache.get(ssgCacheKey)
@@ -1307,7 +1315,7 @@ export default class Server {
 
     // If we're here, that means data is missing or it's stale.
     const maybeCoalesceInvoke = ssgCacheKey
-      ? (fn: any) => withCoalescedInvoke(fn).bind(null, ssgCacheKey, [])
+      ? (fn: any) => withCoalescedInvoke(fn).bind(null, ssgCacheKey!, [])
       : (fn: any) => async () => {
           const value = await fn()
           return { isOrigin: true, value }

packages/next/next-server/server/render.tsx

@@ -637,6 +637,12 @@ export async function renderToHTML(
       }
 
       if (data.unstable_notFound) {
+        if (pathname === '/404') {
+          throw new Error(
+            `The /404 page can not return unstable_notFound in "getStaticProps", please remove it to continue!`
+          )
+        }
+
         ;(renderOpts as any).ssgNotFound = true
         ;(renderOpts as any).revalidate = false
         return null

test/integration/i18n-support/pages/404.js

+export default function NotFound(props) {
+  return (
+    <>
+      <h1 id="not-found">This page could not be found | 404</h1>
+      <p id="prop">{JSON.stringify(props)}</p>
+    </>
+  )
+}
+
+export const getStaticProps = () => {
+  return {
+    props: {
+      is404: true,
+    },
+  }
+}

test/integration/i18n-support/test/index.test.js

@@ -79,6 +79,11 @@ function runTests(isDev) {
           initialRevalidateSeconds: false,
           srcRoute: null,
         },
+        '/404': {
+          dataRoute: `/_next/data/${buildId}/404.json`,
+          initialRevalidateSeconds: false,
+          srcRoute: null,
+        },
         '/en-US/gsp/fallback/first': {
           dataRoute: `/_next/data/${buildId}/en-US/gsp/fallback/first.json`,
           initialRevalidateSeconds: false,

yarn.lock

@@ -15826,9 +15826,10 @@ styled-jsx-plugin-postcss@2.0.1:
     postcss "^7.0.2"
     postcss-load-plugins "^2.3.0"
 
-styled-jsx@3.3.0:
-  version "3.3.0"
-  resolved "https://registry.yarnpkg.com/styled-jsx/-/styled-jsx-3.3.0.tgz#32335c1a3ecfc923ba4f9c056eeb3d4699006b09"
+styled-jsx@3.3.1:
+  version "3.3.1"
+  resolved "https://registry.yarnpkg.com/styled-jsx/-/styled-jsx-3.3.1.tgz#d79f306c42c99cefbe8e76f35dad8100dc5c9ecc"
+  integrity sha512-RhW71t3k95E3g7Zq3lEBk+kmf+p4ZME7c5tfsYf9M5mq6CgIvFXkbvhawL2gWriXLRlMyKAYACE89Qa2JnTqUw==
   dependencies:
     "@babel/types" "7.8.3"
     babel-plugin-syntax-jsx "6.18.0"