Trailing where clause for extension of non-generic type

The solution was simple to switch the order in the extension clause:

extension UIViewController where Self: KeyboardNotificationDelegate

should be

extension KeyboardNotificationDelegate where Self: UIViewController

extension Foo where ... can only be used if Foo is

1. a generic class or structure: extend with a default implementation for generics conforming to some type constraint,
2. a protocol containing some associated types, extend with a default implementation for when an associated type conform to some type constraint
3. a protocol where we extend with a default implementation for when Self is of a specific (object/reference) type, or conforms to some type constraint.

E.g.

// 1class Foo<T> { }extension Foo where T: IntegerType {}struct Foz<T> {}extension Foz where T: IntegerType {}// 2protocol Bar {    associatedtype T}extension Bar where T: IntegerType {}// 3protocol Baz {}extension Baz where Self: IntegerType {}class Bax<T>: Baz {}extension Baz where Self: Bax<Int> {    func foo() { print("foo") }}let a = Bax<Int>()a.foo() // foo

In your case, UIViewController is a non-generic class type, which does not conform to any of the two above.

As you've written in your own answer, the solution is to extend your delegate protocol with a default implementation for cases where Self: UIViewController, rather than attempting to extend UIViewController.