Why private is unaccessible to the extension?

Question

Here is my ViewController.swift file:

class ViewController: UIViewController {
    private var myInt = 10
}

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

Although as mentioned in the Swift documentation - Access Levels section:

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.

Since Swift 4 has been released, I assume that I am able to implement such a thing (it is also mentioned in: What's New in Swift - WWDC 2017 session), however, the complier shows me:

'myInt' is inaccessible due to 'private' protection level

Is it incompatible with what mentioned in the documentation?

As a simple quick solution, I could declare it as:

fileprivate var myInt = 10

but I wonder why is it behaves like this, am I misunderstand what mentioned in the documentation? or is it a "Xcode" bug (used 9.0.1 version)?

Remark: The project has been created in the older Xcode 8 and then migrated to Xcode 9.

Answer 1

In Swift 4, private members are accessible to extensions of that declaration that are in the same file, see SE-0169 – Improve Interaction Between private Declarations and Extensions.

If the project has been created in Xcode 8 (with Swift 3) then Xcode 9 will open it in "Swift 3 mode" and set the "Swift Language Version" to "Swift 3.2". Therefore the stricter Swift 3 restrictions hold.

To make the private extension visible to extension in the same file, set the Swift language version to 4 in the build settings. (Of course that might make more changes in your code necessary.)

Even better, use "Edit -> Convert -> To Current Swift Syntax ..."