Error when exporting to `static` directory (#20969)

Fixes #20408
Fixes #20925

errors/can-not-output-to-static.md

+# Cannot output to /static
+
+#### Why This Error Occurred
+
+Either you set `distDir` to `static` in your `next.config.js` or during `next export` you tried to export to the `static` directory.
+
+This is not allowed due to `static` being a special folder in Next.js used to serve static assets.
+
+#### Possible Ways to Fix It
+
+Use a different `distDir` or export to a different folder.
+
+### Useful Links
+
+- [Static file serving docs](https://nextjs.org/docs/basic-features/static-file-serving)

packages/next/export/index.ts

@@ -252,6 +252,12 @@ export default async function exportApp(
       )
     }
 
+    if (outDir === join(dir, 'static')) {
+      throw new Error(
+        `The 'static' directory is reserved in Next.js and can not be used as the export out directory. https://err.sh/vercel/next.js/can-not-output-to-static`
+      )
+    }
+
     await recursiveDelete(join(outDir))
     await promises.mkdir(join(outDir, '_next', buildId), { recursive: true })
 

test/integration/errors-on-output-to-public/test/index.test.js

@@ -13,7 +13,7 @@ describe('Errors on output to public', () => {
     await fs.writeFile(nextConfig, `module.exports = { distDir: 'public' }`)
     const results = await nextBuild(appDir, [], { stdout: true, stderr: true })
     expect(results.stdout + results.stderr).toMatch(
-      /The 'public' directory is reserved in Next.js and can not be set as/
+      /The 'public' directory is reserved in Next\.js and can not be set as/
     )
     await fs.remove(nextConfig)
   })
@@ -31,7 +31,7 @@ describe('Errors on output to public', () => {
       }
     )
     expect(results.stdout + results.stderr).toMatch(
-      /The 'public' directory is reserved in Next.js and can not be used as/
+      /The 'public' directory is reserved in Next\.js and can not be used as/
     )
   })
 })

test/integration/errors-on-output-to-static/pages/index.js

+export default () => 'hi'

test/integration/errors-on-output-to-static/test/index.test.js

+/* eslint-env jest */
+
+import path from 'path'
+import fs from 'fs-extra'
+import { nextBuild, nextExport } from 'next-test-utils'
+
+jest.setTimeout(1000 * 60 * 1)
+const appDir = path.join(__dirname, '..')
+const nextConfig = path.join(appDir, 'next.config.js')
+
+describe('Errors on output to static', () => {
+  it('Throws error when export out dir is static', async () => {
+    await fs.remove(nextConfig)
+    await nextBuild(appDir)
+    const outdir = path.join(appDir, 'static')
+    const results = await nextExport(
+      appDir,
+      { outdir },
+      {
+        stdout: true,
+        stderr: true,
+      }
+    )
+    expect(results.stdout + results.stderr).toMatch(
+      /The 'static' directory is reserved in Next\.js and can not be used as/
+    )
+  })
+})