import Cocoa
class MsPiggy: NSObject {
    let k = Kermit()
    
    func party() {
        // `Type of expression is ambiguous without more context`
        // but in Swift 5 the compiler correctly sees that both 
        // functions `a` don't match because the keypath is not 
        // writable (`private(set)`). The error message could be better
        k.a(\.color) { v in }
    }
}
class Kermit: NSObject {
    @objc dynamic private(set) var color: NSColor!
}
extension NSObjectProtocol where Self: NSObject {
    func a<Value>(_ keyPath: ReferenceWritableKeyPath<Self, Value>, onChange: @escaping (Value) -> ()) {}
    func a<Value>(_ keyPath: ReferenceWritableKeyPath<Self, Value>, onChange: @escaping (Self, Value) -> ()) {}
}