struct GenericThing <Param1, Param2> {
    init (closure: (String)->()) {
        
    }
}

struct ThingHolder <Param1> {
    func acceptThing <Param2> (thingGenerator: ()->GenericThing<Param1, Param2>) {

    }
}

struct A { }

func demo <Param1> (thingHolder: ThingHolder<Param1>) {
    thingHolder.acceptThing {
        GenericThing<Param1, A> { string in
            // Delete this line and type `string.` and observe that the only option you get is "self" - all other code completion options are gone.
        }
    }
}