Fix asPath of rewrite without basePath (#15760)

Fixes https://github.com/vercel/next.js/issues/15755

packages/next/client/index.js

@@ -9,7 +9,7 @@ import { RouterContext } from '../next-server/lib/router-context'
 import { isDynamicRoute } from '../next-server/lib/router/utils/is-dynamic'
 import * as envConfig from '../next-server/lib/runtime-config'
 import { getURL, loadGetInitialProps, ST } from '../next-server/lib/utils'
-import { delBasePath } from '../next-server/lib/router/router'
+import { hasBasePath, delBasePath } from '../next-server/lib/router/router'
 import initHeadManager from './head-manager'
 import PageLoader from './page-loader'
 import measureWebVitals from './performance-relayer'
@@ -52,15 +52,7 @@ envConfig.setConfig({
 let asPath = getURL()
 
 // make sure not to attempt stripping basePath for 404s
-if (
-  page !== '/404' &&
-  !(
-    page === '/_error' &&
-    hydrateProps &&
-    hydrateProps.pageProps &&
-    hydrateProps.pageProps.statusCode === 404
-  )
-) {
+if (hasBasePath(asPath)) {
   asPath = delBasePath(asPath)
 }
 

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

@@ -30,6 +30,10 @@ function buildCancellationError() {
   })
 }
 
+export function hasBasePath(path: string): boolean {
+  return path === basePath || path.startsWith(basePath + '/')
+}
+
 export function addBasePath(path: string): string {
   return basePath
     ? path === '/'
@@ -269,7 +273,6 @@ export default class Router implements BaseRouter {
       if (as.substr(0, 2) !== '//') {
         // in order for `e.state` to work on the `onpopstate` event
         // we have to register the initial route upon initialization
-
         this.changeState(
           'replaceState',
           formatWithValidation({ pathname: addBasePath(pathname), query }),
@@ -450,7 +453,7 @@ export default class Router implements BaseRouter {
       this.abortComponentLoad(this._inFlightRoute)
     }
 
-    const cleanedAs = delBasePath(as)
+    const cleanedAs = hasBasePath(as) ? delBasePath(as) : as
     this._inFlightRoute = as
 
     // If the url change is only related to a hash change

test/integration/basepath/test/index.test.js

@@ -141,6 +141,38 @@ const runTests = (context, dev = false) => {
     expect(html).toContain('getServerSideProps')
   })
 
+  it('should have correct asPath for rewrite without basePath', async () => {
+    const browser = await webdriver(context.appPort, '/rewrite-no-basePath')
+    expect(await browser.eval(() => window.location.pathname)).toBe(
+      '/rewrite-no-basePath'
+    )
+    expect(await browser.eval(() => window.next.router.asPath)).toBe(
+      '/rewrite-no-basePath'
+    )
+    expect(await browser.eval(() => window.next.router.pathname)).toBe('/gssp')
+  })
+
+  it('should have correct asPath for rewrite without basePath on back()', async () => {
+    const browser = await webdriver(context.appPort, '/rewrite-no-basePath')
+    await browser.eval(() => (window.navigationMarker = true))
+    await browser.eval(() => window.next.router.push('/hello'))
+    await check(
+      () => browser.eval(() => window.location.pathname),
+      '/docs/hello'
+    )
+    await browser.back()
+    await check(
+      () => browser.eval(() => window.location.pathname),
+      '/rewrite-no-basePath'
+    )
+    await check(
+      () => browser.eval(() => window.next.router.asPath),
+      '/rewrite-no-basePath'
+    )
+    expect(await browser.eval(() => window.next.router.pathname)).toBe('/gssp')
+    expect(await browser.eval(() => window.navigationMarker)).toBe(true)
+  })
+
   it('should redirect with basePath by default', async () => {
     const res = await fetchViaHTTP(
       context.appPort,
@@ -216,6 +248,27 @@ const runTests = (context, dev = false) => {
     expect(pathname).toBe('/missing')
   })
 
+  it('should handle 404 urls that start with basePath', async () => {
+    const browser = await webdriver(context.appPort, '/docshello')
+    expect(await browser.eval(() => window.next.router.asPath)).toBe(
+      '/docshello'
+    )
+    expect(await browser.eval(() => window.location.pathname)).toBe(
+      '/docshello'
+    )
+  })
+
+  it('should navigating back to a non-basepath 404 that starts with basepath', async () => {
+    const browser = await webdriver(context.appPort, '/docshello')
+    await browser.eval(() => window.next.router.push('/hello'))
+    await browser.waitForElementByCss('#pathname')
+    await browser.back()
+    check(() => browser.eval(() => window.location.pathname), '/docshello')
+    expect(await browser.eval(() => window.next.router.asPath)).toBe(
+      '/docshello'
+    )
+  })
+
   it('should update dynamic params after mount correctly', async () => {
     const browser = await webdriver(context.appPort, '/docs/hello-dynamic')
     const text = await browser.elementByCss('#slug').text()