cat.swift

protocol CatProtocol {
  associatedtype Food
  func difficult<T>(to: T.Type, food: Food, me: Self) -> Self
}

class Cat : CatProtocol {
  typealias Food = String
  required init() {}
  func difficult<T>(to: T.Type, food: String, me: Cat) -> Self {
    fatalError()
  }
}

class AnyCatBase<Food> {
  func difficult<T>(to: T.Type, food: Food, me: AnyCatBase<Food>) -> Self { fatalError() }
}

final class AnyCatBox<Base: CatProtocol> : AnyCatBase<Base.Food> {
  typealias Food = Base.Food
  var _base: Base
  init(_ base: Base) { _base = base }
  override func difficult<T>(to: T.Type, food: Base.Food, me: AnyCatBase<Base.Food>) -> Self {
    return type(of: self).init(_base.difficult(to: to, food: food, me: _base))
  }
}

struct AnyCat<F> : CatProtocol {
  typealias Food = F
  var _box: AnyCatBase<Food>

  init<Base: CatProtocol>(_ base: Base) where F == Base.Food {
    _box = AnyCatBox(base)
  }

  private init(_ box: AnyCatBase<Food>) {
    _box = box
  }

  func difficult<T>(to: T.Type, food: Food, me: AnyCat<Food>) -> AnyCat<Food> {
    return AnyCat(_box.difficult(to: to, food: food, me: me._box))
  }
}