1. el-select的multiple初始值设置为false单选
2. 选中性别
3. 点击按钮切换multiple为true，选择框的值selectValue设置为[]，但是页面上选择框的值并没有清空
   操作
   
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   实际结果：
   
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   理想结果
   
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  <!-- 选择框 -->
  <el-select v-model="selectValue" :multiple="multiple">
    <el-option v-for="el in sexOptions" :key="el.key" :label="el.label" :value="el.value"></el-option>
  </el-select>
  <!-- 修改选择框类型，单选/多选 -->
  <el-button type="primary" @click="changeSelectType">切换为多选</el-button>

export default {
  data() {
    return {
      multiple: false,
      selectValue: undefined,
      sexOptions: [{ key: 'woman', value: 'woman', label: '女' }, { key: 'man', value: 'man', label: '男' }]
    }
  },
  methods: {
    changeSelectType() {
      // 如果当前选择框为单选，切换为多选，selectValue置为空数组[]
      this.multiple = true
      this.selectValue = []
    }
  }
}

解决办法，先把selectValue置为undefined, 再在nextTick里面切换multiple为true，把selectValue设置为[]

changeSelectType() {
      // 先把selectValue置为undefined
      // 再在nextTick里面切换multiple为true，把selectValue设置为[]
      this.selectValue = undefined
      this.$nextTick(() => {
        this.multiple = true
        this.selectValue = []
      })
    }