function promiseOne() {
  return Promise.resolve(1)
}
    
const promisedOne = promiseOne()
    
// note PromiseLike instead of Promise, this lets it work on any thenable
type ThenArg = T extends PromiseLike ? U : T
    
type PromiseOneThenArg = ThenArg // => number
// or
type PromiseOneThenArg2 = ThenArg> // => number