Allow overloading of ALLOWED_EXPORTS

[Xananax]

Goals

Allow pages to return custom named exports in a well typed manner

Alternatively, at least not trigger an error

Non-Goals

No response

Background

Currently, trying to export a custom named export when using the workspace typescript triggers an error:

import type { Metadata } from 'next'

export const metadata: Metadata = {
  title: 'some title',
  description: '...'
}

export interface PrivateData{
  someValue: number
}

export const privateData: PrivateData = {
  someValue: 100,
}
 

export default function Page() {
  return <h1>Hello, Home page!</h1>
}

Using a comment directive doesn't work:

There seems to be no way to not get errors, which forces me to revert to my default typescript version. Then, my custom export is valid, but I need to explicitly type metadata and lose on the Next-specific checks.

Proposal

I suggest to either:

1. Make sure //@ts-ignore or others work with the code located at packages/next/src/server/typescript/rules/config.ts
2. Better, allow to override this configuration in next.config.js
3. Or, even better, allow to specify new exports by overloading the type in an ambient declaration (might require Type annotation for all exports in the module microsoft/TypeScript#38511)

I have no idea how extending Typescript works and if either suggestion is even possible at all, so I'm not sure if what I'm proposing is sound.

[Xananax]

Note that the workaround proposed in the thread does not work either:

import * as __SELF__ from './page';
__SELF__ satisfies { privateData: PrivateData, metadata: Metadata };

Works to do a check (the satisfies does indeed pass or not pass), but the error on the exported value remains.