Property: Assignment of inherited object regression

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
  width: 640
  height: 480
  visible: true
  title: qsTr("Hello World")

  MyDialog {
    id: dlg
  }

  PopupOpener {
    popup: dlg
  }
}

import QtQuick.Controls 2.12

Dialog {

  contentItem: Label {
    text: "Hello"
  }
}

import QtQuick 2.12
import QtQuick.Controls 2.12

Button {

  property Popup popup;    // error in 5.15: qrc:/main.qml:15:5: Unable to assign MyDialog_QMLTYPE_11 to Popup_QMLTYPE_1
//  property Dialog popup; // ok

  onClicked: popup.open()
  text: "Open Popup"
}