class MyObject: NSObject {
    var foo: Int = 0
    var bar: String = ""
    var baz: NSAttributedStringKey?
}

let x = MyObject()
x.observe(\.foo) { (x, y) in print(x, y) } // => fatal error: Could not extract a String from KeyPath Swift.ReferenceWritableKeyPath<__lldb_expr_55.MyObject, Swift.Int>
// => no execution after this point
x.observe(\.bar) { (x, y) in print(x, y) }
x.observe(\.baz) { (x, y) in print(x, y) }
x.foo = 5
x.bar = "blah"
x.baz = .link