What problem does this feature solve?

When using template, slots with only comment or no content rendered will be ignored and the component will render the default contents.

When using h may lose this feature.

I think this may not be a bug, because render-function needs to be managed all things manually.

But currently is no api that can determine whether the contents of slots are effective.

This is a reappear example: https://codesandbox.io/embed/goofy-mcnulty-lume2?fontsize=14&hidenavigation=1&theme=dark

What does the proposed API look like?

import { isEffectiveSlot } from 'vue'

isEffectiveSlot(slot.default)

If there is already such an api, is my negligence.

You can just loop through the slot nodes and check if they are all comment nodes:

import { Comment } from 'vue'

const isEffectiveSlot = vnodes => vnodes.some(vnode => vnode.type !== Comment)