Fix root route optional catch-all prerendering (#14986)

Fixes https://github.com/vercel/next.js/issues/14964

packages/next/build/index.ts

@@ -791,7 +791,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
       const isSsgFallback = ssgFallbackPages.has(page)
       const isDynamic = isDynamicRoute(page)
       const hasAmp = hybridAmpPages.has(page)
-      let file = normalizePagePath(page)
+      const file = normalizePagePath(page)
 
       // The dynamic version of SSG pages are only prerendered if the fallback
       // is enabled. Below, we handle the specific prerenders of these.
@@ -826,11 +826,12 @@ export default async function build(dir: string, conf = null): Promise<void> {
           // `getStaticPaths` (additionalSsgPaths).
           const extraRoutes = additionalSsgPaths.get(page) || []
           for (const route of extraRoutes) {
-            await moveExportedPage(page, route, route, true, 'html')
-            await moveExportedPage(page, route, route, true, 'json')
+            const pageFile = normalizePagePath(route)
+            await moveExportedPage(page, route, pageFile, true, 'html')
+            await moveExportedPage(page, route, pageFile, true, 'json')
 
             if (hasAmp) {
-              const ampPage = `${normalizePagePath(route)}.amp`
+              const ampPage = `${pageFile}.amp`
               await moveExportedPage(page, ampPage, ampPage, true, 'html')
               await moveExportedPage(page, ampPage, ampPage, true, 'json')
             }

test/integration/dynamic-optional-routing-root-static-paths/next.config.js

+module.exports = {}

test/integration/dynamic-optional-routing-root-static-paths/pages/[[...optionalName]].js

+export const getStaticPaths = () => ({
+  paths: [
+    { params: { optionalName: [] } },
+    { params: { optionalName: ['one'] } },
+    { params: { optionalName: ['one', 'two'] } },
+  ],
+  fallback: false,
+})
+
+export const getStaticProps = ({ params }) => ({ props: { params } })
+
+export default function Page({ params }) {
+  return (
+    <div id="success">
+      {params.optionalName ? params.optionalName.join(',') : 'yay'}
+    </div>
+  )
+}

test/integration/dynamic-optional-routing-root-static-paths/test/index.test.js

+/* eslint-env jest */
+
+import cheerio from 'cheerio'
+import fs from 'fs-extra'
+import {
+  findPort,
+  killApp,
+  launchApp,
+  nextBuild,
+  nextStart,
+  renderViaHTTP,
+} from 'next-test-utils'
+import { join } from 'path'
+
+jest.setTimeout(1000 * 60 * 2)
+
+let app
+let appPort
+const appDir = join(__dirname, '../')
+
+function runTests() {
+  it('should render optional catch-all top-level route with no segments', async () => {
+    const html = await renderViaHTTP(appPort, '/')
+    const $ = cheerio.load(html)
+    expect($('#success').text()).toBe('yay')
+  })
+
+  it('should render optional catch-all top-level route with one segment', async () => {
+    const html = await renderViaHTTP(appPort, '/one')
+    const $ = cheerio.load(html)
+    expect($('#success').text()).toBe('one')
+  })
+
+  it('should render optional catch-all top-level route with two segments', async () => {
+    const html = await renderViaHTTP(appPort, '/one/two')
+    const $ = cheerio.load(html)
+    expect($('#success').text()).toBe('one,two')
+  })
+}
+
+const nextConfig = join(appDir, 'next.config.js')
+
+describe('Dynamic Optional Routing', () => {
+  describe('dev mode', () => {
+    beforeAll(async () => {
+      appPort = await findPort()
+      app = await launchApp(appDir, appPort)
+    })
+    afterAll(() => killApp(app))
+
+    runTests()
+  })
+
+  describe('production mode', () => {
+    beforeAll(async () => {
+      const curConfig = await fs.readFile(nextConfig, 'utf8')
+
+      if (curConfig.includes('target')) {
+        await fs.writeFile(nextConfig, `module.exports = {}`)
+      }
+      await nextBuild(appDir)
+
+      appPort = await findPort()
+      app = await nextStart(appDir, appPort)
+    })
+    afterAll(() => killApp(app))
+
+    runTests()
+  })
+
+  describe('serverless mode', () => {
+    let origNextConfig
+
+    beforeAll(async () => {
+      origNextConfig = await fs.readFile(nextConfig, 'utf8')
+      await fs.writeFile(
+        nextConfig,
+        `module.exports = { target: 'serverless' }`
+      )
+
+      await nextBuild(appDir)
+
+      appPort = await findPort()
+      app = await nextStart(appDir, appPort)
+    })
+    afterAll(async () => {
+      await fs.writeFile(nextConfig, origNextConfig)
+      await killApp(app)
+    })
+
+    runTests()
+  })
+})