flowtype-resolving.js

// BEGIN Other type defs
type User = {
  name: string
}
type Organization = {
  name: string,
  active: boolean
}
// END Other type defs

// BEGIN library code
class Id<A> {}
type Ref<A> = A | Id<A>

type _$MakeReferenceTable<O> = $ObjMap<O, <P>(P) => $Subtype<Ref<P>>>
type _$DowncastRefs<O> = $ObjMap<O, <P>($Subtype<Ref<P>>) => Id<P>>
type $RefAt<A: string, O: {}, D: {}> = $ElementType<_$DowncastRefs<$Diff<_$MakeReferenceTable<D>, O>> & O, A>
// END library code
        
type ThreadRefs = {|
  user: User, 
  organization: Organization, 
|}

class Thread<Refs: {}> {
  length: number
  user: $RefAt<'user', Refs, ThreadRefs>
  organization: $RefAt<'organization', Refs, ThreadRefs>
  
  constructor(args: { 
    length: number, 
    user: $RefAt<'user', Refs, ThreadRefs>, 
    organization: $RefAt<'organization', Refs, ThreadRefs> 
  }) {
    this.length = args.length;
    this.user = args.user;
    this.organization = args.organization;
  }
}

const unresolvedThread: Thread<{}> = (null: any)
const userId: Id<User> = unresolvedThread.user;
// const _userId: User = unresolvedThread.user; errors

const resolvedUserThread: Thread<{ user: User }> = (null: any)
const user: User = resolvedUserThread.user;
const orgId: Id<Organization> = resolvedUserThread.organization;
//const _user: Id<User> = resolvedUserThread.user; errors