Intersecting types based on Generic types

[c10b10]

Lets say I have an object type that represents the template of other more specific types. How does one define the more specific type so that the following usecase does not fail?

Here's a failing attempt:

type Data<T1> = {
  loading: boolean,
  error: string,
  fetch: (options: {
    update: (prevResult: T1, moreData: { moreResult: T1, moreVar: Object }) => Object
  }) => void
}

type SpecificData<T1> = Data<T1> & {
  items: Array<string>
}

const f = (data: SpecificData<SpecificData>) => ({
  data,
  loadMore: () => {
    data.fetch({
      update: (prevResult, { moreResult, moreVar }) => ({
        items: [...prevResult.items, ...moreResult.items]
      })
    })
  }
})