在入門學(xué)習(xí)vue的extend。有疑惑：Vue的擴(kuò)展實(shí)例構(gòu)造器模板里面不可以使用vue的實(shí)例數(shù)據(jù)對(duì)象嗎？

<div id="app">
        <sg></sg>
    </div>
    
    <script type="text/javascript">
    var authorExtend = Vue.extend({
        template: "<p><a :href='url' target='_blank'>{{siteName}}-{{msg}}</a></p>",
        data: function() {
            return {
                url: 'https://segmentfault.com',
                siteName: 'sg'
            }
        }
    });
    new authorExtend().$mount('sg');
    var app = new Vue({
        el: "#app",
        data: {
            msg: "1"
        }
    });
    </script>

控制臺(tái)報(bào)錯(cuò)：

Property or method "msg" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property.

所以，Vue實(shí)例的數(shù)據(jù)用不到擴(kuò)展里面，如果想用，應(yīng)該怎么弄？