This is basically what was described in https://bugreports.qt.io/browse/QTBUG-14254 , which was closed, but still valid in 5.9.

1)

Rectangle {
   property alias borderColor: border.color
}

gives:

Invalid alias reference. Unable to find id "border"

2)

Rectangle {
   id: root
   property alias borderColor:  root.border.color
}

or

Item {
   property alias borderColor:  rect.border.color
   Rectangle {
       id: rect
    }
}

gives:

Invalid alias target location: color

 Any reason why this is not supported or should this be considered a bug? If there is a good reason, it should be documented.