vue插槽 prop 是 undefined 的时候，定义后备内容出错是为什么？求大佬举个例子

官方描述：
定义后备内容，用于插槽 prop 是 undefined 的情形：

<current-user v-slot="{ user = { firstName: 'Guest' } }">
  {{ user.firstName }}
</current-user>

我写的：

//父组件
<myvue v-slot="{user={firstName:'sdad'}}">
        xx{{user.firstName}}
</myvue>

//子组件
<template>
    <div>
        <slot v-bind:user="users"></slot>
    </div>
</template>

<script>
export default {
    name: 'myvue',
    props:["user"],
    data(){
        return{
            users:{
                firstName:undefined,
                lastName:'lastname'
            }
        }
    }
}
</script>

然后不知道为什么报错了：

Failed to compile.

./src/App.vue
Module Error (from ./node_modules/eslint-loader/index.js):
error: Parsing error: Line 1: Unexpected token

> 1 | 0({user={firstName:'sdad'}})
    |        ^ (vue/no-parsing-error) at src\App.vue:3:22:
  1 | <template>
  2 |   <div id="app">
> 3 |   <myvue v-slot="{user={firstName:'sdad'}}">
    |                       ^
  4 |       xx{{user.firstName}}
  5 |   </myvue>
  6 |     <img alt="Vue logo" src="./assets/logo.png">


1 error found.

父组件
 

<template>
    <div>
        <!-- 定义后备内容，用于插槽prop是undefined的情形 -->
        <Test v-slot="{ user = { firstName:'chen' } }">
            {{ user.firstName }}
        </Test>
    </div>
</template>
<script>
import Test from './Test.vue'
export default {
    components:{
        Test
    },
    data(){
        return {
        }

    }
}
</script>

子组件

<template>
    <span>
        <slot :user = "user"></slot>
    </span>
</template>
<script>
export default {
    name: 'Test',
    data(){
        return {
            user: undefined
        }
    }
}
</script>