[Internationalization] Redirect with `{ locale: false }` and dynamic isn't working properly.

Bug report

Describe the bug

Redirect with { locale: false } isn’t working properly.

Context

Before describe the bug, I need to give you some context about what problem I’m trying to solve.

I recently switch my website default locale from pt to en. It means that some old URLs I had localized, now would be inverted, for example:

URL example:

Before: /blog/e-o-coronavirus-hein (default lang PT)
After: /pt/blog/e-o-coronavirus-hein (Default lang EN)

The problem

What’s is happening now is I can’t just setup this redirect via next.config.js or via vercel.json (but I’ll focus on next config).

The way I would try to solve that is setting up something like this:

module.exports = {
  async redirects() {
    return [
      {
        source: "/blog/e-o-coronavirus-hein",
        destination: "/pt/blog/e-o-coronavirus-hein",
        permanent: true,
      },
    ];
  },
};

This won’t work and that’s comprehensible. I assume when I try to access /blog/e-o-coronavirus-hein, since my browser is in English, it’ll try to resolve the request locale to /en. But the stranger thing is that the redirect works but it concatenates everything and I’m being redirected to: /en/pt/blog/e-o-coronavirus-hein.

That’s weird but makes sense because I’m embedding the locale in the path.

In the docs it says we can turn off the locale process, that should do the job, right?

module.exports = {
  async redirects() {
    return [
      {
        source: "/blog/e-o-coronavirus-hein",
        destination: "/pt/blog/e-o-coronavirus-hein",
        locale: false,
        permanent: true,
      },
    ];
  },
};

But now if I try to access /blog/e-o-coronavirus-hein, instead being redirected to /pt/blog/e-o-coronavirus-hein, I just reach 404.

I assume that somehow it’s thinking that’s is a valid route (because I use pages/blog/[slug].js) and it does not consider the redirect setup.

Another problem (which I can open another issue if you want to) is when I try to redirect from a url which even does not exist from an existing one:

module.exports = {
  async redirects() {
    return [
      {
        source: "/2018/08/09/coronavirus",
        destination: "/pt/blog/e-o-coronavirus-hein",
        locale: false,
        permanent: true,
      },
    ];
  },
};

In this case I also get 404.

To Reproduce

I prepare a demo site with some deeper explanation how to test this problem but:

1. Access: https://next-redirect-bug.vercel.app/
2. Check the Defective redirects section
3. Click in both links which should redirect

Expected behavior

As a dev user, I want to be able to setup a redirect from a /blog/some-post to /<locale>/blog/some-post .

Screenshots

Not applicable

System information

• OS: Pop!_OS
• Browser (if applies) [e.g. chrome, safari]: any
• Version of Next.js: [e.g. 10.0.1]: 10.0.3-canary.3
• Version of Node.js: [e.g. 12.0.0]: 12.19.1
• Deployment: [e.g. next start, next export, Vercel, other platform]: Vercel

Additional context

• Repository: https://github.com/raulfdm/next-redirect-bug
• Related: #19302
• https://github.com/vercel/next.js/pull/19766

Update

• This also does not work with rewrites

[robertcoopercode]

I think this might also affect rewrites in addition to redirects since I’m having a similar issue: https://github.com/vercel/next.js/discussions/21096

[c7nj7n]

Despite the traction it does not seem that the issue will be addressed any time soon. Is there a recommended workaround to be able to “cleanly” deal with this? Thank you in advance for your help 😃