fix i18n data pathname resolving

packages/next/src/server/lib/router-utils/resolve-routes.ts

@@ -26,6 +26,7 @@ import { pathHasPrefix } from '../../../shared/lib/router/utils/path-has-prefix'
 import { detectDomainLocale } from '../../../shared/lib/i18n/detect-domain-locale'
 import { normalizeLocalePath } from '../../../shared/lib/i18n/normalize-locale-path'
 import { removePathPrefix } from '../../../shared/lib/router/utils/remove-path-prefix'
+import { getNextPathnameInfo } from '../../../shared/lib/router/utils/get-next-pathname-info'
 import { NextDataPathnameNormalizer } from '../../normalizers/request/next-data'
 import { BasePathPathnameNormalizer } from '../../normalizers/request/base-path'
 import { PostponedPathnameNormalizer } from '../../normalizers/request/postponed'
@@ -178,54 +179,27 @@ export function getResolveRoutes(
       | ReturnType<typeof normalizeLocalePath>
       | undefined = undefined
 
-    if (config.i18n) {
-      const hadTrailingSlash = parsedUrl.pathname?.endsWith('/')
-      const hadBasePath = pathHasPrefix(
-        parsedUrl.pathname || '',
-        config.basePath
-      )
-      initialLocaleResult = normalizeLocalePath(
-        removePathPrefix(parsedUrl.pathname || '/', config.basePath),
-        config.i18n.locales
-      )
+    // TODO: This empty pathname fallback mirrors existing handling in this file but it seems
+    // incorrect that we would ever have an empty pathname here.
+    const pathnameInfo = getNextPathnameInfo(parsedUrl.pathname || '', {
+      nextConfig: config,
+    })
 
+    if (config.i18n) {
       domainLocale = detectDomainLocale(
         config.i18n.domains,
         getHostname(parsedUrl, req.headers)
       )
       defaultLocale = domainLocale?.defaultLocale || config.i18n.defaultLocale
-
-      parsedUrl.query.__nextDefaultLocale = defaultLocale
-      parsedUrl.query.__nextLocale =
-        initialLocaleResult.detectedLocale || defaultLocale
-
-      // ensure locale is present for resolving routes
-      if (
-        !initialLocaleResult.detectedLocale &&
-        !initialLocaleResult.pathname.startsWith('/_next/')
-      ) {
-        parsedUrl.pathname = addPathPrefix(
-          initialLocaleResult.pathname === '/'
-            ? `/${defaultLocale}`
-            : addPathPrefix(
-                initialLocaleResult.pathname || '',
-                `/${defaultLocale}`
-              ),
-          hadBasePath ? config.basePath : ''
-        )
-
-        if (hadTrailingSlash) {
-          parsedUrl.pathname = maybeAddTrailingSlash(parsedUrl.pathname)
-        }
-      }
+      parsedUrl.query.__nextLocale = pathnameInfo.locale
     }
 
     const checkLocaleApi = (pathname: string) => {
       if (
         config.i18n &&
         pathname === urlNoQuery &&
-        initialLocaleResult?.detectedLocale &&
-        pathHasPrefix(initialLocaleResult.pathname, '/api')
+        pathnameInfo.locale &&
+        pathHasPrefix(pathnameInfo.pathname, '/api')
       ) {
         return true
       }

test/e2e/i18n-navigations-middleware/i18n-data-route.test.ts

@@ -0,0 +1,50 @@
+import { nextTestSetup } from 'e2e-utils'
+
+describe('i18n-navigations-middleware', () => {
+  const { next } = nextTestSetup({
+    files: __dirname,
+  })
+
+  it('should initially render the default locale', async () => {
+    const browser = await next.browser('/')
+    expect(await browser.elementById('current-locale').text()).toBe(
+      'Current locale: en'
+    )
+  })
+
+  it('should respect selected locale when navigating to a dynamic route', async () => {
+    const browser = await next.browser('/')
+    // change to "de" locale
+    await browser.elementByCss("[href='/de']").click()
+    const dynamicLink = await browser.waitForElementByCss(
+      "[href='/de/dynamic/1']"
+    )
+    expect(await browser.elementById('current-locale').text()).toBe(
+      'Current locale: de'
+    )
+
+    // navigate to dynamic route
+    await dynamicLink.click()
+
+    // the locale should still be "de"
+    expect(await browser.elementById('dynamic-locale').text()).toBe(
+      'Locale: de'
+    )
+  })
+
+  it('should respect selected locale when navigating to a static route', async () => {
+    const browser = await next.browser('/')
+    // change to "de" locale
+    await browser.elementByCss("[href='/de']").click()
+    const staticLink = await browser.waitForElementByCss("[href='/de/static']")
+    expect(await browser.elementById('current-locale').text()).toBe(
+      'Current locale: de'
+    )
+
+    // navigate to static route
+    await staticLink.click()
+
+    // the locale should still be "de"
+    expect(await browser.elementById('static-locale').text()).toBe('Locale: de')
+  })
+})

test/e2e/i18n-navigations-middleware/middleware.js

@@ -0,0 +1,6 @@
+import { NextResponse } from 'next/server'
+
+export const config = { matcher: ['/foo'] }
+export async function middleware(req) {
+  return NextResponse.next()
+}

test/e2e/i18n-navigations-middleware/next.config.js

@@ -0,0 +1,9 @@
+/**
+ * @type {import('next').NextConfig}
+ */
+module.exports = {
+  i18n: {
+    defaultLocale: 'default',
+    locales: ['default', 'en', 'de'],
+  },
+}

test/e2e/i18n-navigations-middleware/pages/dynamic/[id].js

@@ -0,0 +1,11 @@
+export const getServerSideProps = async ({ locale }) => {
+  return {
+    props: {
+      locale,
+    },
+  }
+}
+
+export default function Dynamic({ locale }) {
+  return <div id="dynamic-locale">Locale: {locale}</div>
+}

test/e2e/i18n-navigations-middleware/pages/index.js

@@ -0,0 +1,37 @@
+import Link from 'next/link'
+
+export const getServerSideProps = async ({ locale }) => {
+  return {
+    props: {
+      locale,
+    },
+  }
+}
+
+export default function Home({ locale }) {
+  return (
+    <main
+      style={{
+        display: 'flex',
+        flexDirection: 'column',
+        gap: '20px',
+      }}
+    >
+      <p id="current-locale">Current locale: {locale}</p>
+      Locale switch:
+      <Link href="/" locale="default">
+        Default
+      </Link>
+      <Link href="/" locale="en">
+        English
+      </Link>
+      <Link href="/" locale="de">
+        German
+      </Link>
+      <br />
+      Test links:
+      <Link href="/dynamic/1">Dynamic 1</Link>
+      <Link href="/static">Static</Link>
+    </main>
+  )
+}

test/e2e/i18n-navigations-middleware/pages/static.js

@@ -0,0 +1,11 @@
+export const getServerSideProps = async ({ locale }) => {
+  return {
+    props: {
+      locale,
+    },
+  }
+}
+
+export default function Static({ locale }) {
+  return <div id="static-locale">Locale: {locale}</div>
+}