Svelte 5: error with a slot prop directly referenced

[mquandalle]

Describe the bug

I have 2 components:

• a <BikeItem bike="" /> that accept a bike prop
• a <FancyList items=[]> that accept an items prop and a slot slot="item"

The following code works with Svelte 4 but not with Svelte 5:

<script>
	import FancyList from "./FancyList.svelte"
	import BikeItem from "./BikeItem.svelte"

	const bikes = ['brompton', 'canyon']
</script>

<FancyList items={bikes}>
	<BikeItem slot="item" let:item={bike} {bike} />
</FancyList>

Svelte 4 REPL

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAE51QQWrDMBD8itgefDHx3bUN7aEQ6A-iHBxlTUVtSUibUiP090pyY4cGSulJq93ZmdnxMMgRHdQHD6qfEGp4MgZKoNmkj_vAkTD-nb5YkTqNE1Ya6rjiJCejLbGXXon5VTpig9UT47Cr1tZuYeBwg3-W77gnnDb4tXOLjnihVSQ9xaFjLTsUp4g3pFVRskL0ao7Vkaum2iypZjMjI6FrfV4P2W-zKrtRU8shQTiwEalO5QIO7PupukS-EnYxhkmf5SDxDDXZC4ZyTW1F_TU7_MxZROnF6OOPQ_wD9uJtmbHe5SLkI5J3lmS3A3yeZsO-SnvhN6_XFP5hNQVz5zSndS94DF8HX7LpXgIAAA==

Logs

No response

System Info

Svelte 5

Severity

annoyance

[dimfeld]

Starting to look into this... it appears that this is only a problem when it's a Svelte component on the slot, not a regular HTML element.

I wrote a test for this based on the component-slot-let-named test, where the template looks like this:

<Nested {things}>
	<SlotInner slot="foo" let:thing {thing} />
</Nested>

The above fails with the same error in the original issue description. If I change it to this:

<Nested {things}>
	<div slot="foo" let:thing>{thing}</div>
</Nested>

Then it works. In the working version, the generated server code contains the line const thing = $$slotProps.thing;, which is absent in the first version. Similar for the client code.

It should just be a matter of rendering that line in the first case as well. Seems a bit tricky so far though that may just be my lack of familiarity with the Svelte 5 parser overall.