import Cocoa

class MsPiggy: NSObject {
    let k = Kermit()
    
    func party() {
        // Here I get `Type of expression is ambiguous without more context`
        // but only with the Swift 5 compiler. In Swift 4.2 mode everything is fine.
        k.a(\.color) { v in }
    }
}

class Kermit: NSObject {
    // If I remove the `private(set)` here, even Swift 5 is happily compiling this
    @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) -> ()) {}
}