记一个大坑，如果 vue 编译提示安装 core-js，很有可能是这个错

WARNING: We noticed you're using the `useBuiltIns` option without declaring a core-js version. Currently, we assume version 2.x when no version is passed. Since this default version will likely change in future versions of Babel, we recommend explicitly setting the core-js version you are using via the `corejs` option.

You should also be sure that the version you pass to the `corejs` option matches the version specified in your `package.json`'s `dependencies` section. If it doesn't, you need to run one of the following commands:

  npm install --save core-js@2    npm install --save core-js@3
  yarn add core-js@2              yarn add core-js@3

我这几两天才学的 vue，用的 vue cli v3.6 版本，然后编译卡住不动，报这个错，因为当时刚好练习嵌套路由，以为是啥 module 和 router 模块起冲突，真的搞了我一下午，我 google 了半天，关于这个报错的相关数据很少，根本找不到答案，最后仔细看了下代码，就是<template>标签没有根 div，真的是血的教训</template>