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    Function

    requireSupportedLocale

    • Narrows a route's locale param to a locale the application serves, or ends the request as a 404.

      Parameters

      • locale: string

        The locale route param, unvalidated

      Returns "en" | "pt" | "es"

      The same value, narrowed to a supported locale

      The [locale] segment accepts one path segment of anything, and the proxy that would normally validate it does not see every request: its matcher excludes any path containing a dot, which is what keeps favicon.ico from being redirected to /en/favicon.ico. A path such as /manifest.json therefore reaches the router unvalidated and matches src/app/[locale]/page.tsx with locale = "manifest.json".

      generateMetadata runs before the layout's own hasLocale guard, so without this the first thing to see that segment is shareMetadata, which throws — logging an error on every such request for a response that was always going to be a 404 anyway.

      shareMetadata keeps throwing on purpose, and this guard is why that is safe: for a real route an unsupported locale is a programming error, and a silent fallback there would publish a wrong canonical URL to crawlers rather than fail loudly. The fix is to never reach it with a bad locale.

      Call it as the first statement of every generateMetadata under [locale]. The layout's separate hasLocale check in its body stays: it guards direct SSG builds, which never call generateMetadata.

      The notFound() control-flow signal when the locale is not one the routing configuration declares; Next resolves it to the nearest not-found boundary

      export async function generateMetadata({ params }: Props): Promise<Metadata> {
        const { locale } = await params;
        requireSupportedLocale(locale);
        // ...
      }

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