What problem does this feature solve?

There are some times in component composition when we need the ability to determine whether a slot has been passed down by the user of the component with some content. In Vue 2 we could check this easily:

<div v-if="this.$slots.mySlot">
   <p>Some unrelated content to the slot or extra markup</p>
   <slot name="mySlot" />
</div>

With the addition of fragments and the changes to the slot system in Vue 3, this simple truthy/falsy check is no longer possible since a slot function will return a truthy object when containing empty text, comments, fragments, etc.

Vue internals are checking this here: https://github.com/vuejs/vue-next/blob/8610e1c9e23a4316f76fb35eebbab4ad48566fbf/packages/runtime-core/src/helpers/renderSlot.ts#L59

However, there is no official way to do this on the user end that I have found that works 100% of the time without having to tap into undocumented internal API.

Some folks are suggesting checking against the symbol in vnode.type, which works in theory however it seems that when compiled these symbols usually evaluate to undefined. It also creates a very hard-to-maintain series of if checks to try to determine the type of vnode that is being received, along with a combination of checks on its `children.

There was an attempt to request this early in January, but sadly it was closed and dismissed with some answers that don't seem to fully solve the problem. #3056

What does the proposed API look like?

The dream would be to expose some sort of slotIsEmpty (naming?) function through the vue package, similarly to how we are getting isRef, etc. that would simplify solving this particular problem.