Function
The incoming request
A 404 rewrite when no route serves the path, otherwise undefined
The catch-all under [locale] already renders the localized "Page not
found" state by calling notFound(). What it cannot do is set the status.
The locale segment has a loading.tsx, so every page under it is wrapped in
a Suspense boundary; the server must commit to 200 OK before it can start
streaming the shell, and by the time notFound() throws, the status has
been sent. Next.js documents this and compensates by injecting
<meta name="robots" content="noindex"> — a soft 404. Removing the loading
skeleton restores the status and was measured to do so, but that skeleton is
its own requirement.
The documented way to keep both is to decide before anything streams, which is here. The rewrite points at the same URL, so the catch-all still renders the same localized page; only the status changes.
Runs after sessionGate, so a personal route still redirects to sign-in
rather than answering 404 to a signed-out learner, and before localize,
which would otherwise commit the response first. A request whose first
segment is not a configured locale is left alone: localize redirects it to
one, and the rewritten path comes back through here.
Spec: lesson-view-polish § "An unknown path under a supported locale renders a localized Page Not Found".
Answers a request for a path no route serves with a real 404.