fix(next/router): Prevent query delete in routing when next.config basePath option is truthy

packages/next/shared/lib/router/router.ts

@@ -1552,7 +1552,11 @@ export default class Router implements BaseRouter {
           query = Object.assign({}, routeInfo.query || {}, query)
         }
 
-        if (routeMatch && pathname !== parsed.pathname) {
+        const cleanedParsedPathname = hasBasePath(parsed.pathname)
+          ? removeBasePath(parsed.pathname)
+          : parsed.pathname
+
+        if (routeMatch && pathname !== cleanedParsedPathname) {
           Object.keys(routeMatch).forEach((key) => {
             if (routeMatch && query[key] === routeMatch[key]) {
               delete query[key]

test/e2e/middleware-base-path/app/pages/dynamic-routes/[routeName].js

@@ -0,0 +1,11 @@
+import { useRouter } from 'next/router'
+
+export default function DynamicRoutes() {
+  const { query } = useRouter()
+
+  return (
+    <main>
+      <h1 id="route-name">{query.routeName}</h1>
+    </main>
+  )
+}

test/e2e/middleware-base-path/app/pages/index.js

@@ -27,6 +27,11 @@ export default function Main({ message }) {
             <a>redirect me to about</a>
           </Link>
         </li>
+        <li>
+          <Link href="/dynamic-routes/hello-world">
+            <a id="go-to-hello-world-anchor">Hello World</a>
+          </Link>
+        </li>
       </ul>
     </div>
   )

test/e2e/middleware-base-path/test/index.test.ts

@@ -35,4 +35,14 @@ describe('Middleware base tests', () => {
     const $ = cheerio.load(html)
     expect($('.title').text()).toBe('About Page')
   })
+  it('router.query must exist when Link clicked page routing', async () => {
+    const browser = await webdriver(next.url, '/root')
+    try {
+      await browser.elementById('go-to-hello-world-anchor').click()
+      const routeName = await browser.elementById('route-name').text()
+      expect(routeName).toMatch('hello-world')
+    } finally {
+      await browser.close()
+    }
+  })
 })