import Foundation

class MyObject: NSObject {
  @objc dynamic var value: Int = 0
}

class MyHostObject: NSObject {
  @objc dynamic let myObject = MyObject()
  var _observation: NSKeyValueObservation?

  override init() {
    super.init()
    _observation = observe(\.myObject.value, options: [.new]) { (myObject, change) in
      print("value changed to \(change.newValue!)")
    }
  }

  deinit {
    // `myObject` is in Limbo right now, but I can still call it's methods and, hopefully, remove observation
    _observation?.invalidate()
    _observation = nil
  }
}

var myHostObject: MyHostObject? = MyHostObject()
myHostObject = nil // crashing here with:
// *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'An instance 0x101c10290 of class issue_NSKeyValueObservation.MyHostObject was deallocated while key value observers were still registered with it. Current observation info: <NSKeyValueObservationInfo 0x101c14ed0> (
<NSKeyValueObservance 0x101c14c20: Observer: 0x101c115f0, Key path: myObject.value, Options: <New: YES, Old: NO, Prior: NO> Context: 0x0, Property: 0x101c129b0>
)'