Can't create identifiers in inline component event handler

import QtQuick 2.15
import QtQuick.Window 2.15

Window {
  width: 640; height: 480; visible: true
  title: qsTr("Inline component problem")

  component TestComponent : MouseArea {
    onClicked: {
      const testVar = 1
      // let testVar = 2
    }
  }

  TestComponent {
    anchors.fill: parent
  }
}

QQmlApplicationEngine failed to load component
qrc:/main.qml:10:13: Identifier testVar has already been declared