Could not find a declaration file for module

Just installed as a dev dependency in react 19 and getting:

```
Could not find a declaration file for module 'react-polymorphic-types'. '/Users/me/node_modules/react-polymorphic-types/empty.js' implicitly has an 'any' type.
There are types at '/Users/me/node_modules/react-polymorphic-types/index.d.ts', but this result could not be resolved when respecting package.json "exports". The 'react-polymorphic-types' library may need to update its package.json or typings.ts(7016)
```

Start by reproducing the React 19 TypeScript error and inspect package.json alongside index.d.ts and empty.js to see how the package exports are resolved. Done means the declaration file is found without the TS7016 error when the package is installed as a development dependency.

Written by the indexing model from the issue text.