Protocol type constraint issue

issue.swift

import Cocoa

protocol ThingId { }

protocol Thing { 
  func getId<A: ThingId>() -> A
}

struct MyThingId: ThingId { }

struct MyThing: Thing {
    func getId<A: ThingId>() -> A {
        return (MyThingId() as? A)!
    }
}

let aThing = MyThing()
aThing.getId()

In a Playground this errors on line 18 with a
Cannot invoke 'getId' with no arguments

I'm not sure if I'm just using generics wrong with protocols but it seems inconsistent that Swift uses type parameters in classes/structs and associated types in protocols for what amounts to the same thing – allowing to defer the definition of the type. This different version works as I intended the above to work:

import Cocoa

protocol ThingId { }

protocol Thing {
    typealias IdType
    func getId() -> IdType
}

struct MyThingId: ThingId { }

struct MyThing: Thing {
    typealias IdType = MyThingId
    func getId() -> MyThingId {
        return MyThingId()
    }
}

let aThing = MyThing()
aThing.getId()