swift – 为什么私有扩展无法访问？

class ViewController: UIViewController {
    private var myInt = 10
}

extension ViewController {
    func printMyInt() {
        print(myInt)
    }
}

虽然如Swift documentation – Access Levels section中所述：

Private access restricts the use of an entity to the enclosing
declaration,and to extensions of that declaration that are in the
same file. Use private access to hide the implementation details of a
specific piece of functionality when those details are used only
within a single declaration.

由于Swift 4已经发布,我认为我能够实现这样的东西(它也在What’s New in Swift – WWDC 2017会话中提到),然而,编译器告诉我：

‘myInt’ is inaccessible due to ‘private’ protection level

它与文档中提到的内容不兼容吗？

作为一个简单的快速解决方案,我可以将其声明为：

fileprivate var myInt = 10

但我想知道为什么它会像这样,我误解了文档中提到的内容？或者它是一个“Xcode”错误(使用9.0.1版本)？

备注：该项目已在较旧的Xcode 8中创建,然后迁移到Xcode 9.