Q_GADGET types not handled properly when passed as QML function arguments

#include <QGuiApplication>
#include <QQuickView>
#include <QQmlEngine>
#include <QQmlContext>
#include <QStandardItemModel>
#include <QDebug>

class Something : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QJSValue theFunction READ function WRITE setFunction NOTIFY functionChanged)
    Q_PROPERTY(QJSValue theRectFunction READ rectFunction WRITE setRectFunction NOTIFY rectFunctionChanged)
public:
    Something(QObject* = 0)
    {
    }

    void setFunction(const QJSValue& function)
    {
      if (!mFunction.equals(function))
      {
        mFunction = function;
        Q_ASSERT(mFunction.isCallable());
        Q_EMIT functionChanged();
      }
    }

    QJSValue function() const
    {
      return mFunction;
    }

    void setRectFunction(const QJSValue& function)
    {
      if (!mRectFunction.equals(function))
      {
        mRectFunction = function;
        Q_ASSERT(mRectFunction.isCallable());
        Q_EMIT rectFunctionChanged();
      }
    }

    QJSValue rectFunction() const
    {
      return mRectFunction;
    }

    Q_INVOKABLE void callFunction(const QModelIndex& idx)
    {
        qDebug() << "Something::callFunction" << idx;
        QJSValueList args = QJSValueList() << qjsEngine(this)->toScriptValue(idx);
        mFunction.call(args).toVariant();
    }

    Q_INVOKABLE void callRectFunction(const QRect& rect)
    {
        qDebug() << "Something::callRectFunction" << rect;
        QJSValueList args = QJSValueList() << qjsEngine(this)->toScriptValue(rect);
        mRectFunction.call(args).toVariant();
    }


Q_SIGNALS:
  void functionChanged();
  void rectFunctionChanged();

private:
    mutable QJSValue mFunction;
    mutable QJSValue mRectFunction;
};

int main(int argc, char** argv)
{
    QGuiApplication app(argc, argv);

    QStandardItemModel model;
    model.appendRow(new QStandardItem("First"));
    model.appendRow(new QStandardItem("Second"));

    qmlRegisterType<Something>("Things", 1, 0, "Something");

    QQuickView view;
    view.engine()->rootContext()->setContextProperty("_model", &model);
    view.setSource(QUrl("main.qml"));
    view.show();

    return app.exec();
}

#include "main.moc"

import QtQuick 2.3

import Things 1.0

Rectangle {
    id: root
    width: 560
    height: 360

    Something {
        id: st
        theFunction: function(idx) {
            console.log("theFunction idx", idx);
            console.log("theFunction row", idx.row);
            return 42;
        }

        theRectFunction: function(rect) {
            console.log("theFunction rect", rect);
            console.log("theFunction rectWidth", rect.width);
        }
    }

    Text {
        id: textElement
        text: "Click"
        width: 200
        height: 30
        MouseArea {
            anchors.fill: parent
            onClicked: {
                var idx = _model.index(0, 0);
                console.log("onClicked: idx", idx);
                console.log("onClicked: row", idx.row);
                st.callFunction(idx);

                var rect = Qt.rect(50, 50, 100, 100);
                console.log("onClicked: rect", rect)
                console.log("onClicked: rectWidth", rect.width)
                st.callRectFunction(rect)
            }
        }
    }
}

$ ./QTBUG-48073.app/Contents/MacOS/QTBUG-48073 
qml: onClicked: idx QModelIndex(0,0,0x7ff18a5733e0,QStandardItemModel(0x7fff5aed4880))
qml: onClicked: row 0
Something::callFunction QModelIndex(0,0,0x7ff18a5733e0,QStandardItemModel(0x7fff5aed4880))
qml: theFunction idx QVariant(QModelIndex)
qml: theFunction row undefined
qml: onClicked: rect QRectF(50, 50, 100, 100)
qml: onClicked: rectWidth 100
Something::callRectFunction QRect(50,50 100x100)
qml: theFunction rect QVariant(QRect)
qml: theFunction rectWidth undefined