Typescript Conditional Type Returns

type InnerType<T> = T extends Promise<infer U> ? U : never;

export function CaptureErr<
  F extends (...args: any[]) => any,
  N extends string,
  R extends ReturnType<F>,
>(
  name: N,
  fn: F,
  message?: string,
): R extends Promise<any> ? Promise<InnerType<R> | Err<N>> : R | Err<N> {
  try {
    const result = fn();
    if (result instanceof Promise) {
      // Type 'Promise<Err<N> | InnerType<R>>' is not assignable to type
      // 'R extends Promise<any> ? Promise<InnerType<R> | Err<N>> : R | Err<N>'.
      return result.catch((e: Error) => {
        e.cause = new Err(name ?? e.name, message ?? e.message);
        return Promise.resolve(e as Err<N>);
      }) as Promise<InnerType<R> | Err<N>>;
    }
    return result;
  } catch (error) {
    error.cause = new Err(name ?? error.name, message ?? error.message);
    return error;
  }
}