test(vercel#36855): add test case

test/integration/export-404-html/.gitignore

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+.next-dev

test/integration/export-404-html/next.config.js

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+module.exports = (phase) => {
+  return {
+    trailingSlash: false,
+  }
+}

test/integration/export-404-html/pages/index.js

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+export default () => <div id="home-page" />

test/integration/export-404-html/test/index.test.js

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+/* eslint-env jest */
+
+import { join } from 'path'
+import { nextBuild, nextExport, File } from 'next-test-utils'
+
+import { promises } from 'fs'
+
+const { access, stat } = promises
+const appDir = join(__dirname, '../')
+const outdir = join(appDir, 'out')
+const context = {}
+context.appDir = appDir
+const nextConfig = new File(join(appDir, 'next.config.js'))
+
+const fileExist = (path) =>
+  access(path)
+    .then(() => stat(path))
+    .then((stats) => (stats.isFile() ? true : false))
+    .catch(() => false)
+
+// Issue #36855
+// https://github.com/vercel/next.js/issues/36855
+describe('Static 404 Export', () => {
+  it('only export 404.html when trailingSlash: false', async () => {
+    await nextBuild(appDir)
+    await nextExport(appDir, { outdir })
+
+    expect(await fileExist(join(outdir, '404.html'))).toBe(true)
+    expect(await fileExist(join(outdir, '404.html.html'))).toBe(false)
+    expect(await fileExist(join(outdir, '404/index.html'))).toBe(false)
+  })
+
+  it('export 404.html and 404/index.html when trailingSlash: true', async () => {
+    nextConfig.replace(`trailingSlash: false`, `trailingSlash: true`)
+    await nextBuild(appDir)
+    await nextExport(appDir, { outdir })
+    nextConfig.restore()
+
+    expect(await fileExist(join(outdir, '404/index.html'))).toBe(true)
+    expect(await fileExist(join(outdir, '404.html.html'))).toBe(false)
+    expect(await fileExist(join(outdir, '404.html'))).toBe(true)
+  })
+})