Use Svelte Component as Slot

I trying to build a new Svelte application.
So, I have the following Main.svelte file:

<script>
    import TitleSlide from '../lib/slides/TitleSlide.svelte'
</script>
<TitleSlide>
    <span slot="title">To infinity and beyond</span>
    <span slot="caption">or story about toys</span>
</TitleSlide>

File TitleSlide.svelte has 2 slots:

<section>
    <h1>
        <slot name="title">
            <span class="missing">Unknown title</span>
        </slot>
    </h1>
    <p class="fragment">
        <slot name="caption"></slot>
    </p>
</section>

It works great, next step I rewrite Main.svelte to this:

<script>
    import TitleSlide from '../lib/slides/TitleSlide.svelte'
    import Title from '../lib/partial/Title.svelte'
    import Caption from '../lib/partial/Caption.svelte'
</script>
<TitleSlide>
    <Title slot="title">To infinity and beyond</Title>
    <Caption slot="caption">or story about toys</Caption>
</TitleSlide>

And simple Title.svelte and Caption.svelte:

<slot>
    <span class="missing">Unknown title/caption</span>
</slot>

But it looks strange, I want to use more simplified syntax like this:

<script>
    import TitleSlide from '../lib/slides/TitleSlide.svelte'
    import Title from '../lib/partial/Title.svelte'
    import Caption from '../lib/partial/Caption.svelte'
</script>
<TitleSlide>
    <Title>To infinity and beyond</Title>
    <Caption>or story about toys</Caption>
</TitleSlide>

So in other words, I want to use Component as Slot, I want to create two Components Title and Caption, and set up a target slot for them.

I'm not sure if it is possible or not

You can't have two default slots. So if you don't like using the somewhat clunking slot naming then I think you've only got two other options. First you can move everything into TitleSlide like this.

Main.svelte would just be:

<script>
    import TitleSlide from '../lib/slides/TitleSlide.svelte'
</script>
<TitleSlide />

and then your TitleSlide.svelte could be (you can obviously pass data into TitleSlide as props for reusing throughout your website, but I'm ignoring that for now):

<script>
    import Title from '../lib/partial/Title.svelte'
    import Caption from '../lib/partial/Caption.svelte'
</script>

<section>
    <h1>
        <Title>To infinity and beyond</Title>
    </h1>
    <p class="fragment">
        <Caption>or story about toys</Caption>
    </p>
</section>

Alternately you could do away with TitleSlide and then your Main.svelte becomes this:

<section>
    <Title>To infinity and beyond</Title>
    <Caption>or story about toys</Caption>
</section>

And then in your individual components you could add the markup that was in TitleSlide. It'd look like this in Title.Svelte

<h1>
    <slot>
        <span class="missing">Unknown title/caption</span>
    </slot>
</h1

This is not possible right now. The only slot that can be assigned without naming it, is the default slot.