Type erasure with multiple adopting types

TypeErasure.swift

// Paste me into a playground!

import Cocoa

//: # Basic Setup

protocol FancyProtocol {
    associatedtype Thing
    func holdPinkyUp(x: Thing)
}

struct Dashing: FancyProtocol {
    func holdPinkyUp(x: String) { print("Dashing: \(x)") }
}

struct Spiffy: FancyProtocol {
   func holdPinkyUp(x: String) { print("Spiffy: \(x)") }
}

//: ## BoxBase
//: The base implements the protocol but everything just fatal errors.
//: It exists to give us an abstraction we can override later
class AnyFancyBoxBase<T>: FancyProtocol {
    func holdPinkyUp(x: T) {
        //never called
        fatalError()
    }
}

//: ## The Box
//: Here we override the BoxBase and specify that our generic parameter
//: implements the protocol. This is just a trampoline that forwards
//: everything to base.
final class _FancyBox<Base: FancyProtocol>: AnyFancyBoxBase<Base.Thing> {
    var base: Base
    init(_ base: Base) {
        self.base = base
    }
    override func holdPinkyUp(x: Base.Thing) {
        base.holdPinkyUp(x: x)
    }
}

//: ## Type-erased wrapper
//: Our type-erased AnyFancy that specifies the base box class,
//: which being a base class frees us from knowing what is inside it.
//:
//: The generic initializer is using FancyProtocol as a generic constraint
//: and thus we satisfy the requirement of using an associated type
struct AnyFancy<T>: FancyProtocol {
    var _box: AnyFancyBoxBase<T>
    func holdPinkyUp(x: T) {
        _box.holdPinkyUp(x: x)
    }
    
    init<U: FancyProtocol>(_ base: U) where U.Thing == T {
        _box = _FancyBox(base)
    }
}

let dashing = Dashing()
let spiffy = Spiffy()

//: ## Magic
//: Our type-erased AnyFancy that specifies the base box class,
//: which being a base class frees us from caring about the implementation
var anyFancy = AnyFancy(dashing)
print("\(type(of: anyFancy))")
anyFancy.holdPinkyUp(x: "ok")

//: Because Spiffy binds FancyProtocol.Thing to String it is compatible
anyFancy = AnyFancy(spiffy)
anyFancy.holdPinkyUp(x: "woo")