API routes don't return with concurrentFeatures

[bduff9]

What version of Next.js are you using?

12.0.2-canary.6

What version of Node.js are you using?

14.17.0

What browser are you using?

Chrome

What operating system are you using?

macOS

How are you deploying your application?

Vercel

Describe the Bug

A freshly generated next app does not seem to allow API routes when concurrentFeatures is turned on. The browser never loads anything and the console displays output similar to the following:

wait  - compiling /api/hello (server only)...
wait  - compiling...
event - compiled successfully in 498 ms (281 modules)
error - Error: Cannot find module for page: /api/hello
wait  - compiling /_error...
wait  - compiling...
event - compiled successfully in 160 ms (284 modules)

Expected Behavior

API routes to not be affected by concurrentFeatures. If I turn them off, the API routes load as expected.

To Reproduce

1. Generate a new next app with npx create-next-app --ts --use-npm
2. cd into the new app directory and edit the next.config.js file to add the concurrentFeatures flag like so:

module.exports = {
  experimental: {
    concurrentFeatures: true
  }
}

3. Save and run npm run dev
4. Try to navigate to localhost:3000/api/hello
5. The page will never load and you will see the following error in the console:

Error: Cannot find module for page: /api/hello

[shuding]

Hey! I tested locally that with the latest next@canary (>= 12.0.4-canary.3) API routes are now working as expected, so this should have been fixed by a previous PR. Also adding #31227 to avoid future regression.

Please let us know if you can still reproduce this issue :)

[balazsorban44]

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