Type annotation is not allowed in signal handler

import QtQuick
Window {
    onWidthChanged: function(): void{ console.log(width) } // error
    function fn(): void { }
} 

In the above code, it seems that type annotation in the signal handler of onWidthChanged is not allowed.

Error compiling qml file: .../Main.qml:4:31: error: Type annotations are not permitted for the return value of JavaScript functions

However, as far as I know, QML allows type annotation and utilizes it for optimzation like the function `fn()`.