How to represent a generic return type of a fetch handler

[mckernanin]

Working on converting a project form using an old generated swagger client, I have a caching function that looks like this:

export async function makeCachedRequest<RT>(
  fn: () => RT,
  key: string[],
  expires?: number,
) {
  const cached = await getJSON<RT>(key.join("-"));
  if (cached) {
    return cached;
  }
  const response = await fn();
  await setJSON(key.join("-"), response, expires);
  return response;
}

get/setJSON are just wrappers around redis,

Implementation looks like this, and it properly passes the response type through:

const cachedOrFetched = await makeCachedRequest(
  async () => await fetch(),
  ["cache", "key"],
  1000
);

I'm trying to implement something similar for caching responses for calls made with openapi-fetch, but am struggling to type the function properly. Currently, I have this:

export async function makeCachedRequestv2<
  RT extends
    | { error: undefined; data: unknown }
    | { error: ClientError; data: undefined },
>(fn: () => Promise<RT>, key: string[], expires?: number) {
  const cached = await getJSON<RT["data"]>(key.join("-"));
  if (cached) {
    return cached;
  }
  const response = await fn();
  if (response.error) {
    throw new EsiClientRequestError(response.error.error);
  }
  await setJSON(key.join("-"), response.data, expires);
  return response.data;
}

Implemented the same way, but with openapi-fetch, the type doesn't narrow down to the actual return type of the request, resulting in unknown being the type of cachedOrFetched when using the second version of the function. Is there a type exported that I can use to represent any (haha) return value from open-api fetch, without requiring any generics to be passed in?

[mckernanin]

One more note, I can use the existing function to cache the response, but it would end up caching an error as well (and without specifying a type for RT to extend, I have no way of making the caching function accept that the error and data properties exist on the response.