Identifiable.swift

import Foundation

protocol Identifiable {
    associatedtype IdentifierType : Equatable
    var id : IdentifierType { get }
}

struct Activity : Identifiable {
    let id : Int
}

struct Thumbnail : Identifiable {
    let id : Int
}

typealias Identifier<T: Identifiable> = T.IdentifierType

struct Cuepoint {
    let thumbnailId : Identifier<Thumbnail>
    let activityId : Identifier<Activity>
}

let thumbnail = Thumbnail(id: 1)
let activity = Activity(id: 123)

let activityCuepoint = Cuepoint(thumbnailId: activity.id, activityId: thumbnail.id)

print(activityCuepoint.activityId == thumbnail.id) // 😒
print(activityCuepoint.thumbnailId == activity.id) // 😞