swift-type-erasure-pattern.swift

protocol CastProtocol {
    func cast<T>(to: T.Type) -> T?
}

protocol CatProtocol : CastProtocol {
    associatedtype Food
    
    var name: String { get set }
    func nya() -> String
    mutating func sleep()
    func findFood() -> Food?
    func eat(food: Food) -> Void
    func clone() -> Self
    func cast<T>(to: T.Type) -> T?
    
    func difficult1<T>(to: T.Type, food: Food)
}

class Cat : CatProtocol {
    required init() {}
    var name: String = "tama"
    func nya() -> String { return "nyaaaa" }
    func sleep() { print("zzz...") }
    func findFood() -> Int? {
        return 3
    }
    func eat(food: Int) { print("yummy \(food)") }
    func clone() -> Self {
        return type(of: self).init()
    }
    func cast<T>(to: T.Type) -> T? {
        return self as? T
    }
    func difficult1<T>(to: T.Type, food: Food) {
    }
}

struct AnyCat1<FD> : CatProtocol {
    init<X>(_ x: X) where X: CatProtocol, X.Food == FD {
        var x = x
        _getName = { x.name }
        _setName = { x.name = $0 }
        _nya = { x.nya() }
        _sleep = { x.sleep() }
        _findFood = { x.findFood() }
        _eat = { x.eat(food: $0) }
        _clone = { AnyCat1(x.clone()) }
        _cast = x
    }
    
    var name: String {
        get { return _getName() }
        set { _setName(newValue) }
    }

    func nya() -> String {
        return _nya()
    }
    
    func sleep() { _sleep() }

    func findFood() -> FD? {
        return _findFood()
    }
    
    func eat(food: FD) -> Void {
        _eat(food)
    }
    
    func clone() -> AnyCat1 {
        return _clone()
    }
    
    func cast<T>(to: T.Type) -> T? {
        return _cast.cast(to: to)
    }

    var _getName: () -> String
    var _setName: (String) -> Void
    var _nya: () -> String
    var _sleep: () -> Void
    var _findFood: () -> FD?
    var _eat: (FD) -> Void
    var _clone: () -> AnyCat1
    var _cast: CastProtocol
}

答え
https://gist.github.com/rintaro/c240100472967973de3f42e16080dfa1

stdlibのAnySequenceもrintaroさんパターン

class _AnySequenceBox<Element> {
  internal func _makeIterator() -> AnyIterator<Element> { _abstract() }
}

final class _SequenceBox<S> : _AnySequenceBox<S.Iterator.Element> {
  internal override func _makeIterator() -> AnyIterator<Element> {
    return AnyIterator(_base.makeIterator())
  }
}

struct AnySequence<Element> : Sequence {
  public init<S : Sequence>(_ base: S)
    where S.Element == Element 
  {
    self._box = _SequenceBox(_base: base)
  }
  internal let _box: _AnySequenceBox<Element>
}

https://github.com/apple/swift/blob/master/stdlib/public/core/ExistentialCollection.swift.gyb