Ensure default _app is used when falling back to default _error

packages/next/client/index.tsx

@@ -468,7 +468,7 @@ async function render(renderingProps: RenderRouteInfo): Promise<void> {
 // 404 and 500 errors are special kind of errors
 // and they are still handle via the main render method.
 function renderError(renderErrorProps: RenderErrorProps): Promise<any> {
-  const { App, err } = renderErrorProps
+  let { App, err } = renderErrorProps
 
   // In development runtime errors are caught by our overlay
   // In production we catch runtime errors using componentDidCatch which will trigger renderError
@@ -497,10 +497,18 @@ function renderError(renderErrorProps: RenderErrorProps): Promise<any> {
     .loadPage('/_error')
     .then(({ page: ErrorComponent, styleSheets }) => {
       return lastAppProps?.Component === ErrorComponent
-        ? import('../pages/_error').then((m) => ({
-            ErrorComponent: m.default as React.ComponentType<{}>,
-            styleSheets: [],
-          }))
+        ? import('../pages/_error')
+            .then((errorModule) => {
+              return import('../pages/_app').then((appModule) => {
+                App = appModule.default as any as AppComponent
+                renderErrorProps.App = App
+                return errorModule
+              })
+            })
+            .then((m) => ({
+              ErrorComponent: m.default as React.ComponentType<{}>,
+              styleSheets: [],
+            }))
         : { ErrorComponent, styleSheets }
     })
     .then(({ ErrorComponent, styleSheets }) => {

test/production/fatal-render-errror/app/pages/_app.js

@@ -0,0 +1,15 @@
+export default function App({ Component, pageProps }) {
+  if (process.env.NODE_ENV === 'production' && typeof window !== 'undefined') {
+    if (!window.renderAttempts) {
+      window.renderAttempts = 0
+    }
+    window.renderAttempts++
+    throw new Error('error in custom _app')
+  }
+  return (
+    <>
+      <p id="custom-app">from _app</p>
+      <Component {...pageProps} />
+    </>
+  )
+}

test/production/fatal-render-errror/app/pages/_error.js

@@ -0,0 +1,6 @@
+export default function Error() {
+  if (process.env.NODE_ENV === 'production' && typeof window !== 'undefined') {
+    throw new Error('error in custom _app')
+  }
+  return <div>Error encountered!</div>
+}

test/production/fatal-render-errror/app/pages/index.js

@@ -0,0 +1,3 @@
+export default function Page() {
+  return <p>index page</p>
+}

test/production/fatal-render-errror/app/pages/with-error.js

@@ -0,0 +1,6 @@
+export default function Error() {
+  if (process.env.NODE_ENV === 'production' && typeof window !== 'undefined') {
+    throw new Error('error in pages/with-error')
+  }
+  return <div>with-error</div>
+}

test/production/fatal-render-errror/index.test.ts

@@ -0,0 +1,55 @@
+import { createNext, FileRef } from 'e2e-utils'
+import { NextInstance } from 'test/lib/next-modes/base'
+import { check, renderViaHTTP, waitFor } from 'next-test-utils'
+import webdriver from 'next-webdriver'
+import { join } from 'path'
+
+describe('fatal-render-errror', () => {
+  let next: NextInstance
+
+  beforeAll(async () => {
+    next = await createNext({
+      files: new FileRef(join(__dirname, 'app')),
+      dependencies: {},
+    })
+  })
+  afterAll(() => next.destroy())
+
+  it('should render page without error correctly', async () => {
+    const html = await renderViaHTTP(next.url, '/')
+    expect(html).toContain('index page')
+    expect(html).toContain('from _app')
+  })
+
+  it('should handle fatal error in _app and _error without loop on direct visit', async () => {
+    const browser = await webdriver(next.url, '/with-error')
+
+    // wait a bit to see if we are rendering multiple times unexpectedly
+    await waitFor(500)
+    expect(await browser.eval('window.renderAttempts')).toBeLessThan(10)
+
+    const html = await browser.eval('document.documentElement.innerHTML')
+    expect(html).not.toContain('from _app')
+    expect(html).toContain(
+      'Application error: a client-side exception has occurred'
+    )
+  })
+
+  it('should handle fatal error in _app and _error without loop on client-transition', async () => {
+    const browser = await webdriver(next.url, '/')
+    await browser.eval('window.renderAttempts = 0')
+
+    await browser.eval('window.next.router.push("/with-error")')
+    await check(() => browser.eval('location.pathname'), '/with-error')
+
+    // wait a bit to see if we are rendering multiple times unexpectedly
+    await waitFor(500)
+    expect(await browser.eval('window.renderAttempts')).toBeLessThan(10)
+
+    const html = await browser.eval('document.documentElement.innerHTML')
+    expect(html).not.toContain('from _app')
+    expect(html).toContain(
+      'Application error: a client-side exception has occurred'
+    )
+  })
+})