Problem

• When trying to generate DTS files for a specific module that depends on another module having it's types declared via module augmentation (declare module ...) results in an error.
• However, TypeScript compiler is able to resolve augmented modules.

Version info

• tsup 8.3.0
• typescript 5.6.3

Repro

1. Create a new package called tsup-augmented-module via npm init and put inside index.d.ts module augmentation, e.g.:

declare module "tsup-augmented-module" {

    export type SetupOptions = {
        token: string;
        someOtherOption?: number;
    };

}

2. Specify "types": "index.d.ts" in package.json.
3. Create another consumer module via npm init and put your augmented module as dependency:

"dependencies": {
  "tsup-augmented-module": "file:../tsup-augmented-module",
}

4. Now, in your index.ts write:

import { SetupOptions } from 'tsup-augmented-module';

export function test(options: SetupOptions): string {
  return 'test accomplished!';
}

5. Now, run npx tsup --entry index.ts --dts-only --dts-resolve and see an error like:

RollupError: "SetupOptions" is not exported by 'tsup-augmented-module', imported by "./index.ts".

Expectation

As typescript complier is able to resolve augmented modules, tsup should be able to as well.