protocol _PSPDFKVOController: class {}
extension _PSPDFKVOController {
    func observe2<Value>(_ object: AnyObject, keyPath: KeyPath<Self, Value>, options: NSKeyValueObservingOptions = [], block: @escaping (NSKeyValueObservedChange<Value>) -> Void) -> NSKeyValueObservation? {
        let controller = self as! PSPDFKVOController<Value>
        controller.observe(controller, "", options: options) { (a, b, c) in // Error: Extra argument in call
            // WAT?
        }
}