Update fallback locale for locale: false (#19322)

This updates the fallback locale for `locale: false` to be the `defaultLocale` instead of the currently active `locale` as it allows passing through URLs more seamlessly as a URL without the locale prefixed can be treated as the `defaultLocale` instead of having to worry if the locale matches the currently active locale. This also ensures `locale={false}` is tested in the i18n-support-catchall suite

Closes: https://github.com/vercel/next.js/issues/19048

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

@@ -616,7 +616,10 @@ export default class Router implements BaseRouter {
     let localeChange = options.locale !== this.locale
 
     if (process.env.__NEXT_I18N_SUPPORT) {
-      this.locale = options.locale || this.locale
+      this.locale =
+        options.locale === false
+          ? this.defaultLocale
+          : options.locale || this.locale
 
       if (typeof options.locale === 'undefined') {
         options.locale = this.locale
@@ -627,12 +630,10 @@ export default class Router implements BaseRouter {
       } = require('../i18n/normalize-locale-path') as typeof import('../i18n/normalize-locale-path')
 
       const parsedAs = parseRelativeUrl(hasBasePath(as) ? delBasePath(as) : as)
-
       const localePathResult = normalizeLocalePath(
         parsedAs.pathname,
         this.locales
       )
-
       if (localePathResult.detectedLocale) {
         this.locale = localePathResult.detectedLocale
         url = addBasePath(localePathResult.pathname)
@@ -1235,7 +1236,7 @@ export default class Router implements BaseRouter {
           this.locales
         )
         parsedAs.pathname = localePathResult.pathname
-        options.locale = localePathResult.detectedLocale || options.locale
+        options.locale = localePathResult.detectedLocale || this.defaultLocale
         asPath = formatWithValidation(parsedAs)
       }
     }

test/integration/i18n-support-base-path/pages/locale-false.js

@@ -42,6 +42,14 @@ export default function Page(props) {
         <a id="to-gssp-slug">to /gssp/first</a>
       </Link>
       <br />
+      <Link href={`/gssp/first`} locale={false}>
+        <a id="to-gssp-slug-default">to /gssp/first (default locale)</a>
+      </Link>
+      <br />
+      <Link href={`/gsp`} locale={false}>
+        <a id="to-gsp-default">to /gsp (default locale)</a>
+      </Link>
+      <br />
     </>
   )
 }

test/integration/i18n-support-catchall/pages/[[...slug]].js

@@ -35,12 +35,12 @@ export default function Page(props) {
       </Link>
       <br />
 
-      <Link href="/" locale="fr">
+      <Link href="/fr" locale={false}>
         <a id="to-fr-locale-index">to /fr</a>
       </Link>
       <br />
 
-      <Link href="/another" locale="fr">
+      <Link href="/fr/another" locale={false}>
         <a id="to-fr-locale-another">to /fr/another</a>
       </Link>
       <br />

test/integration/i18n-support/pages/locale-false.js

@@ -42,6 +42,14 @@ export default function Page(props) {
         <a id="to-gssp-slug">to /gssp/first</a>
       </Link>
       <br />
+      <Link href={`/gssp/first`} locale={false}>
+        <a id="to-gssp-slug-default">to /gssp/first (default locale)</a>
+      </Link>
+      <br />
+      <Link href={`/gsp`} locale={false}>
+        <a id="to-gsp-default">to /gsp (default locale)</a>
+      </Link>
+      <br />
     </>
   )
 }

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

@@ -27,56 +27,42 @@ async function addDefaultLocaleCookie(browser) {
 }
 
 export function runTests(ctx) {
-  it('should trigger full navigation for unsupported locale', async () => {
-    const browser = await webdriver(ctx.appPort, '/')
+  it('should use default locale when no locale is in href with locale false', async () => {
+    const browser = await webdriver(
+      ctx.appPort,
+      `${ctx.basePath}/nl/locale-false?nextLocale=fr`
+    )
 
     await browser.eval('window.beforeNav = 1')
-    await browser.eval(`(function() {
-      window.next.router.push('/auto-export', '/auto-export', { locale: 'no' })
-    })()`)
-
-    await browser.waitForElementByCss('#not-found')
-    expect(await browser.eval('window.beforeNav')).toBe(null)
-
-    const props = JSON.parse(await browser.elementByCss('#props').text())
-    expect(props.is404).toBe(true)
-    expect(props.locale).toBe('en-US')
-    expect(props.locales).toEqual(locales)
-    expect(props.defaultLocale).toBe('en-US')
-  })
 
-  it('should 404 for non configured locales direct visit', async () => {
-    for (const [pathname, gssp] of [
-      ['/gsp', true],
-      ['/gssp', true],
-      ['/auto-export'],
-      ['/'],
-      ['/gsp/fallback/first'],
-    ]) {
-      const res = await fetchViaHTTP(
-        ctx.appPort,
-        `/no${pathname === '/' ? '' : pathname}`,
-        undefined,
-        {
-          redirect: 'manual',
-        }
-      )
+    if (!ctx.isDev) {
+      await browser.eval(`(function() {
+        document.querySelector('#to-gssp-slug-default').scrollIntoView()
+        document.querySelector('#to-gsp-default').scrollIntoView()
+      })()`)
 
-      expect(res.status).toBe(404)
+      await check(async () => {
+        const hrefs = await browser.eval(`Object.keys(window.next.router.sdc)`)
+        hrefs.sort()
 
-      if (gssp) {
-        const res = await fetchViaHTTP(
-          ctx.appPort,
-          `/_next/data/${ctx.buildId}/no${pathname}.json`,
-          undefined,
-          {
-            redirect: 'manual',
-          }
+        assert.deepEqual(
+          hrefs.map((href) =>
+            new URL(href).pathname
+              .replace(ctx.basePath, '')
+              .replace(/^\/_next\/data\/[^/]+/, '')
+          ),
+          [
+            '/en-US/gsp.json',
+            '/fr/gsp.json',
+            '/fr/gsp/fallback/first.json',
+            '/fr/gsp/fallback/hello.json',
+          ]
         )
-
-        expect(res.status).toBe(404)
-      }
+        return 'yes'
+      }, 'yes')
     }
+
+    expect(await browser.eval('window.beforeNav')).toBe(1)
   })
 
   if (ctx.isDev) {
@@ -802,6 +788,7 @@ export function runTests(ctx) {
               .replace(/^\/_next\/data\/[^/]+/, '')
           ),
           [
+            '/en-US/gsp.json',
             '/fr/gsp.json',
             '/fr/gsp/fallback/first.json',
             '/fr/gsp/fallback/hello.json',