What problem does this feature solve?

Dynamic v-slot:[] can't recieve "'< By here However we have such scenario need to pass function and params to v-slot:[] like:

<template v-slot:[formatSlotName(' before')]>
  <h1>I'm slot 'before'</h1>
</template>

Reproduction Link: https://codesandbox.io/s/intelligent-bell-hkpd6?file=/src/App.vue

What does the proposed API look like?

No new API need. Is it possible to make compatibility by judging current attribute is slot to determine whether to emit error? If so. i am happy to create a PR ^_^

The restriction is by design as HTML attribute names cannot include ", ' and <. We don't want to introduce more caveats between usage of SFC and in-DOM templates. You can use a computed prop as a workaround.

@Justineo Thanks for your explanation. I have a little question. According to the specification. Is the design of v-slots:[slotsName] follows specification by Empty attribute syntax ?

Yes, v-slots:[slotsName] is an attribute name.