protocol_extension_crash.swift

struct NotePage {
  var title: String
  var contents: String
}

class NoteCover {
  var page: NotePage
  
  init(page: NotePage) {
    self.page = page
  }
}

enum DeskItem {
  case note(NoteCover)
}

protocol DeskSellable {
  static func sell(_: DeskItem) -> Self
}

protocol Seller {
  associatedtype TradeType
}

extension Seller where TradeType: DeskSellable {
  func sellItem(_ deskItem: DeskItem) -> TradeType {
    TradeType.sell(deskItem)
  }
}

class DeskSeller: Seller {
  enum TradeType: DeskSellable {
    case sell(DeskItem)
  }
  
  func sellNote(_ originalPage: NotePage) -> TradeType {
    let coveredNote = NoteCover(page: originalPage)
    // not working - causes crash
    return self.sellItem(.note(coveredNote))
    // working without crash
    return TradeType.sell(.note(coveredNote))
  }
}

let deskSeller = DeskSeller()

switch deskSeller.sellNote(.init(title: "hello", contents: "world!")) {
case let .sell(.note(noteCover)):
  print("note page: \(noteCover.page)") // runtime crash point
  break
default:
  break
}