fix basepath trailing slash

packages/next/client/normalize-trailing-slash.ts

@@ -11,7 +11,7 @@ export function removePathTrailingSlash(path: string): string {
  * Normalizes the trailing slash of a path according to the `trailingSlash` option
  * in `next.config.js`.
  */
-const normalizePathTrailingSlash = process.env.__NEXT_TRAILING_SLASH
+export const normalizePathTrailingSlash = process.env.__NEXT_TRAILING_SLASH
   ? (path: string): string => {
       if (/\.[^/]+\/?$/.test(path)) {
         return removePathTrailingSlash(path)

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

@@ -20,12 +20,17 @@ import { parseRelativeUrl } from './utils/parse-relative-url'
 import {
   normalizeTrailingSlash,
   removePathTrailingSlash,
+  normalizePathTrailingSlash,
 } from '../../../client/normalize-trailing-slash'
 
 const basePath = (process.env.__NEXT_ROUTER_BASEPATH as string) || ''
 
 export function addBasePath(path: string): string {
-  return basePath ? (path === '/' ? basePath : basePath + path) : path
+  return basePath
+    ? path === '/'
+      ? normalizePathTrailingSlash(basePath)
+      : basePath + path
+    : path
 }
 
 export function delBasePath(path: string): string {

test/integration/trailing-slashes/next.config.js

@@ -2,4 +2,5 @@ module.exports = {
   experimental: {
     // <placeholder>
   },
+  // basePath: '/docs',
 }

test/integration/trailing-slashes/test/index.test.js

@@ -70,19 +70,19 @@ function testShouldResolve(expectations) {
 function testLinkShouldRewriteTo(expectations) {
   it.each(expectations)(
     '%s should have href %s',
-    async (href, expectedHref) => {
-      const content = await renderViaHTTP(appPort, `/linker?href=${href}`)
+    async (linkPage, expectedHref) => {
+      const content = await renderViaHTTP(appPort, linkPage)
       const $ = cheerio.load(content)
       expect($('#link').attr('href')).toBe(expectedHref)
     }
   )
 
   it.each(expectations)(
     '%s should navigate to %s',
-    async (href, expectedHref) => {
+    async (linkPage, expectedHref) => {
       let browser
       try {
-        browser = await webdriver(appPort, `/linker?href=${href}`)
+        browser = await webdriver(appPort, linkPage)
         await browser.elementByCss('#link').click()
 
         await browser.waitForElementByCss('#hydration-marker')
@@ -97,10 +97,10 @@ function testLinkShouldRewriteTo(expectations) {
 
   it.each(expectations)(
     '%s should push route to %s',
-    async (href, expectedHref) => {
+    async (linkPage, expectedHref) => {
       let browser
       try {
-        browser = await webdriver(appPort, `/linker?href=${href}`)
+        browser = await webdriver(appPort, linkPage)
         await browser.elementByCss('#route-pusher').click()
 
         await browser.waitForElementByCss('#hydration-marker')
@@ -134,13 +134,13 @@ function testWithoutTrailingSlash() {
   ])
 
   testLinkShouldRewriteTo([
-    ['/', '/'],
-    ['/about', '/about'],
-    ['/about/', '/about'],
-    ['/about?hello=world', '/about?hello=world'],
-    ['/about/?hello=world', '/about?hello=world'],
-    ['/catch-all/hello/', '/catch-all/hello'],
-    ['/catch-all/hello.world/', '/catch-all/hello.world'],
+    ['/linker?href=/', '/'],
+    ['/linker?href=/about', '/about'],
+    ['/linker?href=/about/', '/about'],
+    ['/linker?href=/about?hello=world', '/about?hello=world'],
+    ['/linker?href=/about/?hello=world', '/about?hello=world'],
+    ['/linker?href=/catch-all/hello/', '/catch-all/hello'],
+    ['/linker?href=/catch-all/hello.world/', '/catch-all/hello.world'],
   ])
 }
 
@@ -164,13 +164,13 @@ function testWithTrailingSlash() {
   ])
 
   testLinkShouldRewriteTo([
-    ['/', '/'],
-    ['/about', '/about/'],
-    ['/about/', '/about/'],
-    ['/about?hello=world', '/about/?hello=world'],
-    ['/about/?hello=world', '/about/?hello=world'],
-    ['/catch-all/hello/', '/catch-all/hello/'],
-    ['/catch-all/hello.world/', '/catch-all/hello.world'],
+    ['/linker?href=/', '/'],
+    ['/linker?href=/about', '/about/'],
+    ['/linker?href=/about/', '/about/'],
+    ['/linker?href=/about?hello=world', '/about/?hello=world'],
+    ['/linker?href=/about/?hello=world', '/about/?hello=world'],
+    ['/linker?href=/catch-all/hello/', '/catch-all/hello/'],
+    ['/linker?href=/catch-all/hello.world/', '/catch-all/hello.world'],
   ])
 }
 
@@ -318,6 +318,11 @@ describe('Trailing slashes', () => {
       ['/docs/catch-all/hello/world', '/docs/catch-all/hello/world/'],
       ['/docs/catch-all/hello.world/', '/docs/catch-all/hello.world'],
     ])
+
+    testLinkShouldRewriteTo([
+      ['/docs/linker?href=/about', '/docs/about/'],
+      ['/docs/linker?href=/', '/docs/'],
+    ])
   })
 
   describe('production mode, with basepath, trailingSlash: true', () => {
@@ -346,5 +351,10 @@ describe('Trailing slashes', () => {
       ['/docs/catch-all/hello/world', '/docs/catch-all/hello/world/'],
       ['/docs/catch-all/hello.world/', '/docs/catch-all/hello.world'],
     ])
+
+    testLinkShouldRewriteTo([
+      ['/docs/linker?href=/about', '/docs/about/'],
+      ['/docs/linker?href=/', '/docs/'],
+    ])
   })
 })