acemarke.ts

type Action =
  | { type: 'ReturnsPromise' }
  | { type: 'ReturnsUndefined' }

type R<A> = 
  | A extends { type: 'ReturnsPromise' } ? Promise<unknown> : never
  | A extends { type: 'ReturnsUndefined' } ? undefined : never
  | unknown

type Dispatch<A> = (action: A) => R<A>

const dispatch = <A>(action: A) => undefined as R<A>

dispatch({ type: 'ReturnsPromise' as const }).then(() => foo())
// failure: dispatch returns undefined (expected)
dispatch({ type: 'ReturnsUndefined' as const }).then(() => null)