Check for invalid pages

packages/next/build/index.ts

@@ -770,9 +770,8 @@ export default async function build(dir: string, conf = null): Promise<void> {
 
   const analysisEnd = process.hrtime(analysisBegin)
   telemetry.record(
-    eventBuildOptimize({
+    eventBuildOptimize(pagePaths, {
       durationInSeconds: analysisEnd[0],
-      totalPageCount: pagePaths.length,
       staticPageCount: staticPages.size,
       ssrPageCount: pagePaths.length - staticPages.size,
     })

packages/next/telemetry/events/build.ts

@@ -19,13 +19,32 @@ type EventBuildOptimized = {
   totalPageCount: number
   staticPageCount: number
   ssrPageCount: number
+  hasDunderPages: boolean
+  hasTestPages: boolean
 }
 
+const REGEXP_DIRECTORY_DUNDER = /[\\/]__[^\\/]+(?<![\\/]__(?:tests|mocks))__[\\/]/i
+const REGEXP_DIRECTORY_TESTS = /[\\/]__(tests|mocks)__[\\/]/i
+const REGEXP_FILE_TEST = /\.(?:spec|test)\.[^.]+$/i
+
 export function eventBuildOptimize(
-  event: EventBuildOptimized
+  pagePaths: string[],
+  event: Omit<
+    EventBuildOptimized,
+    'totalPageCount' | 'hasDunderPages' | 'hasTestPages'
+  >
 ): { eventName: string; payload: EventBuildOptimized } {
   return {
     eventName: EVENT_BUILD_OPTIMIZE,
-    payload: event,
+    payload: {
+      ...event,
+      totalPageCount: pagePaths.length,
+      hasDunderPages: pagePaths.some(path =>
+        REGEXP_DIRECTORY_DUNDER.test(path)
+      ),
+      hasTestPages: pagePaths.some(
+        path => REGEXP_DIRECTORY_TESTS.test(path) || REGEXP_FILE_TEST.test(path)
+      ),
+    },
   }
 }

test/integration/telemetry/pages/__ytho__/lel.js

@@ -0,0 +1,3 @@
+export default () => {
+  return 'oops, I happen to look like a React component'
+}

test/integration/telemetry/pages/hello.test.js

@@ -0,0 +1 @@
+export default () => 'Hello Test'

test/integration/telemetry/test/index.test.js

@@ -92,6 +92,19 @@ describe('Telemetry CLI', () => {
     expect(stderr2).toMatch(/isSrcDir.*?true/)
   })
 
+  fit('detects tests correctly for `next build`', async () => {
+    const { stderr } = await runNextCommand(['build', appDir], {
+      stderr: true,
+      env: {
+        NEXT_TELEMETRY_DEBUG: 1,
+      },
+    })
+
+    console.log(stderr)
+    expect(stderr).toMatch(/hasDunderPages.*?true/)
+    expect(stderr).toMatch(/hasTestPages.*?true/)
+  })
+
   it('detects isSrcDir dir correctly for `next dev`', async () => {
     let port = await findPort()
     let stderr = ''