a bug of"undefined" when using slot

Vue.js version

2.1.7

Reproduction Link

https://jsfiddle.net/jkin8010/j7s5prkv/5/

Steps to reproduce

Icon.vue

<template>
    <i :class="className"></i>
</template>
<script>
export default {
    props: {
        type: String
    },
    computed: {
        className () {
            return `c-icon c-icon-${this.type}`
        }
    }
}
</script>

Hello.vue

<template>
  <div class="hello">
    <icon type="clear"></icon>
    <slot name="right"><slot>
  </div>
</template>
<script>
import Icon from './icon.vue';
export default {
  components: {
    Icon
  }
}
</script>

App.vue

<template>
  <div id="app">
    <hello>
      test2
    </hello>
  </div>
</template>

<script>
import Hello from './Hello'
export default {
  components: {
    Hello
  }
}
</script>

What is Expected?

What is actually happening?

[posva]

see #6793

[fritx]

With vue@2.4.4/vue-template-compiler@2.4.4 and before, it was fine:

but upgraded them both to the latest: 2.5.0, the weird “undefined” show up again:

Is it bug of element-ui or vue?

Related: https://github.com/vuejs/vue/issues/4993, https://github.com/fritx/vue-at/issues/3