QtQuick compiler treats parameters of a signal handler differently if called arguments

import QtQuick 2.6

Rectangle {
    id: root

    color: "red"
    height: 500
    width: 500

    signal doit(var arguments)

    onDoit: {
        console.warn("json: " + JSON.stringify(arguments));
    }

    MouseArea {
        anchors.fill: parent
        onClicked: {
            doit({item: "test",
                     params: {
                         asdf: "fdsa",
                         x: 4
                     }
                 });
        }
    }
}