[swift-evolution] Enhancing access levels without breaking changes

[Tino Heth]

Imho there is a simple solution to reach the goals of SE-0169 without breaking compatibility:
Just allow extensions inside type declarations.

class MyVC: UIViewController {
    private let numberOfSections = 0

    extension: UITableViewDataSource {
        // Skipping the class and assume we want to extend the surrounding type
        func numberOfSections(in tableView: UITableView) -> Int {
            return numberOfSections
        }

        func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
            return 0
        }
    }

    private extension {
        // this would contain everything that shoudn't be visible for other extensios

        var secret: Int = 0

        public extension MyFriendClass {
            // oh, well, I make an exception here for a trustworthy type
            func checkSecret(of controller: MyVC) -> Bool {
                return controller.secret > 0
            }
        }

        private extension {
            // this is so secret, I'm not even allowed to copy it
        }
    }

    public func myMethod() {
        print("This is just a boring method")
    }
}

It has the downside of shifting code to the right (you could as well leave those extension-blocks unindented), but lots of advantages:
- No change for private needed
- It can be nested as much as you like to satisfy even the most absurd desires of encapsulation
- It reminds me on operator definitions inside type declarations 
- No change for fileprivate needed (but actually, I think there is very little need to keep fileprivate)

I wish this would only be a joke, but writing the example, I actually started liking the concept (but I have a terrible headache right now which might affect my mind) — so either this receives some feedback, or I might start re-proposing this ;-)

- Tino
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