Dangerous behavior of instanceof or ability to assign objects of "different types"

import QtQuick 2.0

Rectangle {
    readonly property bool fakeProperty: false

    implicitWidth: 100
    implicitHeight: 100

    Rectangle {
        anchors.centerIn: parent
        width: 10
        height: 10
        color: "red"
    }
}

import QtQuick 2.0

Rectangle {
    implicitWidth: 100
    implicitHeight: 100

    Rectangle {
        anchors.centerIn: parent
        width: 10
        height: 10
        color: "red"
    }
}

import QtQuick 2.9
import QtQuick.Controls 2.2

ApplicationWindow {
    property SpecialRectangle1 rect1: SpecialRectangle1 {}
    property SpecialRectangle2 rect2: SpecialRectangle2 {}
    //property SpecialRectangle1 rect3: Rectangle {} //You can activate this line if fakeProperty from SpecialRectangle1 is deleted
    property SpecialRectangle2 rect4: Rectangle {}

    visible: true
    width: 640
    height: 480

    Text {
        anchors.centerIn: parent
        text: "%1 %2".arg(rect1 instanceof SpecialRectangle2).arg(rect2 instanceof SpecialRectangle1)
    }
}