I am encountering an issue when building TypeScript declaration files (.d.ts) using tsup. Specifically, the generated exports1.d.ts file contains unexpected content, which causes all types to be lost. Below are the files involved and the problem in detail:

Files:

my-exports.ts

export * as Foo from './exports1';

exports1.ts

export * from 'react/experimental';

Problem:

When building the .d.ts files using tsup, the generated exports1.d.ts file contains the following code:

import * as experimental from 'react/experimental';
export * from 'react/experimental';

function _mergeNamespaces(n, m) {
  m.forEach(function (e) {
    e && typeof e !== 'string' && !Array.isArray(e) && Object.keys(e).forEach(function (k) {
      if (k !== 'default' && !(k in n)) {
        var d = Object.getOwnPropertyDescriptor(e, k);
        Object.defineProperty(n, k, d.get ? d : {
          enumerable: true,
          get: function () { return e[k]; }
        });
      }
    });
  });
  return Object.freeze(n);
}

var exports1 = /*#__PURE__*/_mergeNamespaces({
  __proto__: null
}, [experimental]);

export { exports1 as e };

The function _mergeNamespaces seems to be causing the issue, where all types are lost in the process.

Expected Behavior:

I expect tsup to generate the .d.ts files without the _mergeNamespaces function and without losing any type information. The file should export types correctly, preserving the structure from the original exports1.ts.

Any Idea?