如何从常规扩展返回Swift泛型？

protocol Container {
    associatedtype Item
    
    func addItem(_ item: Item)
    func getItem() -> Item?
}

struct Stack<T>: Container {
    private var items = [T]()
    
    mutating func addItem(_ item: T) {
        items.append(item)
    }
    
    mutating func getItem() -> T? {
        return items.popLast()
    }
}

struct Queue<T>: Container {
    private var items = [T]()
    
    mutating func addItem(_ item: T) {
        items.append(item)
    }
    
    mutating func getItem() -> T? {
        if items.isEmpty {
            return nil
        } else {
            return items.removeFirst()
        }
    }
}

var stack = Stack<Int>()
stack.addItem(1)
stack.addItem(2)
print(stack.getItem()) // 输出 Optional(2)

var queue = Queue<String>()
queue.addItem("Hello")
queue.addItem("World")
print(queue.getItem()) // 输出 Optional("Hello")