Dynamic Routes in Symlinks is not working

Bug report

Describe the bug

Dynamic Routes in symlinks is not working

To Reproduce

I don't think github repos support symlink commits so this is how you can reproduce this:

In project folder:

mkdir pages/nolink
echo "export default function Post() {return <div>Works</div>}" > "pages/nolink/[id].js"
echo "export default function index() {return <div>Works</div>}" > "pages/nolink/index.js"
ln -s nolink pages/symlinktest

• http://localhost:3000/symlinktest/123 shows "404 This page could not be found."
• http://localhost:3000/symlinktest/ shows "works"
• http://localhost:3000/nolink/123 shows "works"
• http://localhost:3000/nolink/ shows "works"

Expected behavior

• http://localhost:3000/symlinktest/123 should show "works"

System information

• OS: Linux
• Version of Next.js: ^9.5.2
• Version of Node.js: v12.18.2

Additional context

I also tried this in next.config.js:

module.exports = {
    webpack: (config, { buildId, dev }) => {
      config.resolve.symlinks = false
      return config
    },
  }

index.js is for debug the error. Removing index.js did not help. This points to Dynamic Routes not supporting symlinks.

Start by reproducing the routes using pages/nolink/[id].js, pages/nolink/index.js, the pages/symlinktest link, and the shown next.config.js webpack setting. Trace how dynamic routes are discovered for symlinked pages, then verify that /symlinktest/123 resolves while the existing non-dynamic routes continue to work.