Proposal: External variable in extension

Currently possible to add extension to class, so we can separate the implementation to different class files. In some situation this still have some limitations,
For example:

public class Racer {

    public var name:String

    public init(name:String) {
        self.name = name
    }
}

public extension Racer {

    public var winTimes:Int {
        // requires access local variable
    }

    public func win() {
        // need to update local variable
    }
}

the method in the extension sometimes need add another variable to store values, if you are extending the system class, that will be very hard to make this happen

So I am thinking, can we add another namespace “external” besides “public, private, internal”, with this we can define a “external var” inside the extension scope.
the variable needs a default value, and only can accessed in the extension scope, the implementation for the example will like this:

public class Racer {

    public var name:String

    public init(name:String) {
        self.name = name
    }
}

public extension Racer {

    external var _winTimes:Int = 0

    public var winTimes:Int {
        return _winTimes
    }

    public func win() {
        _winTimes++
    }
}
<https://github.com/chenyunguiMilook/swift-evolution#impact-on-existing-code&gt;