ignore .d.ts files inside pages folder (#30728)

Fixes #30698 and #30618

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added

Author: thibautsabot

packages/next/build/entries.ts

@@ -36,6 +36,12 @@ export function createPagesMapping(
   }
 ): PagesMapping {
   const previousPages: PagesMapping = {}
+
+  // Do not process .d.ts files inside the `pages` folder
+  pagePaths = extensions.includes('ts')
+    ? pagePaths.filter((pagePath) => !pagePath.endsWith('.d.ts'))
+    : pagePaths
+
   const pages: PagesMapping = pagePaths.reduce(
     (result: PagesMapping, pagePath): PagesMapping => {
       let page = pagePath.replace(

test/integration/404-page/pages/invalidExtension.d.ts

@@ -0,0 +1 @@
+export const container: string

test/integration/404-page/test/index.test.js

@@ -38,6 +38,11 @@ const runTests = (mode = 'server') => {
     expect(res.status).toBe(404)
   })
 
+  it('should use pages/404 for .d.ts file', async () => {
+    const html = await renderViaHTTP(appPort, '/invalidExtension')
+    expect(html).toContain('custom 404 page')
+  })
+
   it('should not error when visited directly', async () => {
     const res = await fetchViaHTTP(appPort, '/404')
     expect(res.status).toBe(404)

test/integration/page-extensions/pages/invalidExtension.d.ts

@@ -0,0 +1 @@
+export const container: string

test/integration/page-extensions/test/index.test.js

@@ -64,4 +64,17 @@ describe('Page Extensions', () => {
       '@zeit/next-typescript is no longer needed since Next.js has built-in support for TypeScript now'
     )
   })
+
+  it('should not throw if .d.ts file inside the pages folder', async () => {
+    await fs.writeFile(
+      nextConfig,
+      `module.exports = { pageExtensions: ['js', 'ts', 'tsx'] }`
+    )
+
+    const { stdout } = await runNextCommand(['build', appDir], { stdout: true })
+
+    await fs.remove(nextConfig)
+
+    expect(stdout).toContain('Compiled successfully')
+  })
 })