Ensure trailingSlash redirect applies correctly for i18n (#19859)

This ensures locales aren't applied for the trailing slash redirect un-necessarily

Fixes: https://github.com/vercel/next.js/issues/19784

packages/next/lib/load-custom-routes.ts

@@ -466,12 +466,14 @@ export default async function loadCustomRoutes(
         destination: '/:file',
         permanent: true,
         locale: config.i18n ? false : undefined,
+        internal: true,
       } as Redirect,
       {
         source: '/:notfile((?!\\.well-known(?:/.*)?)(?:[^/]+/)*[^/\\.]+)',
         destination: '/:notfile/',
         permanent: true,
         locale: config.i18n ? false : undefined,
+        internal: true,
       } as Redirect
     )
     if (config.basePath) {
@@ -481,6 +483,7 @@ export default async function loadCustomRoutes(
         permanent: true,
         basePath: false,
         locale: config.i18n ? false : undefined,
+        internal: true,
       } as Redirect)
     }
   } else {
@@ -489,6 +492,7 @@ export default async function loadCustomRoutes(
       destination: '/:path+',
       permanent: true,
       locale: config.i18n ? false : undefined,
+      internal: true,
     } as Redirect)
     if (config.basePath) {
       redirects.unshift({
@@ -497,6 +501,7 @@ export default async function loadCustomRoutes(
         permanent: true,
         basePath: false,
         locale: config.i18n ? false : undefined,
+        internal: true,
       } as Redirect)
     }
   }

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

@@ -720,6 +720,7 @@ export default class Server {
     const redirects = this.customRoutes.redirects.map((redirect) => {
       const redirectRoute = getCustomRoute(redirect, 'redirect')
       return {
+        internal: redirectRoute.internal,
         type: redirectRoute.type,
         match: redirectRoute.match,
         statusCode: redirectRoute.statusCode,

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

@@ -24,6 +24,7 @@ export type Route = {
   statusCode?: number
   name: string
   requireBasePath?: false
+  internal?: true
   fn: (
     req: IncomingMessage,
     res: ServerResponse,
@@ -197,7 +198,11 @@ export default class Router {
       const activeBasePath = keepBasePath ? this.basePath : ''
 
       if (keepLocale) {
-        if (!localePathResult.detectedLocale && parsedUrl.query.__nextLocale) {
+        if (
+          !testRoute.internal &&
+          parsedUrl.query.__nextLocale &&
+          !localePathResult.detectedLocale
+        ) {
           currentPathname = `${activeBasePath}/${parsedUrl.query.__nextLocale}${
             currentPathnameNoBasePath === '/' ? '' : currentPathnameNoBasePath
           }`

test/integration/custom-routes/test/index.test.js

@@ -644,6 +644,7 @@ const runTests = (isDev = false) => {
             ),
             source: '/:path+/',
             statusCode: 308,
+            internal: true,
           },
           {
             destination: '/:lang/about',

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

@@ -406,6 +406,26 @@ export function runTests(ctx) {
     }
   })
 
+  it('should apply trailingSlash redirect correctly', async () => {
+    for (const [testPath, path, hostname, query] of [
+      ['/first/', '/first', 'localhost', {}],
+      ['/en/', '/en', 'localhost', {}],
+      ['/en/another/', '/en/another', 'localhost', {}],
+      ['/fr/', '/fr', 'localhost', {}],
+      ['/fr/another/', '/fr/another', 'localhost', {}],
+    ]) {
+      const res = await fetchViaHTTP(ctx.appPort, testPath, undefined, {
+        redirect: 'manual',
+      })
+      expect(res.status).toBe(308)
+
+      const parsed = url.parse(res.headers.get('location'), true)
+      expect(parsed.pathname).toBe(path)
+      expect(parsed.hostname).toBe(hostname)
+      expect(parsed.query).toEqual(query)
+    }
+  })
+
   it('should apply redirects correctly', async () => {
     for (const [path, shouldRedirect, locale] of [
       ['/en-US/redirect-1', true],