Svelte 4: forwarded slots ignore the fallback parent slot

[opensas]

Describe the bug

I define a Card.svelte component with a message slot, which has a default fallback content.

I can render the Card component with the fallback slot and also override it correctly ==> OK

I then define a ForwardCard.svelte which calls Card.svelte, customizing it's titlle, and passes on the message slot.

ForwardCard slot can be overriden, but if the slot is not specified IT WON'T fallback to use the default of the parent component, Card.svelte.

svelte repl reproducing the issue: link

<!-- Card.svelte -->
<script>
	export let message = ''
</script>

<div>
	<slot name="title">
		<div>This is a Card.svelte</div>
	</slot>

	<slot name="message" {message}>
		<div>fallback slot: {message}</div>
	</slot>
	
	<hr />
</div>

<!-- ForwardCard.svelte -->
<script>
	import Card from './Card.svelte'

	export let message = 'custom message'
</script>

<Card {message}>
	<div slot="title">This is a ForwardCard.svelte</div>
	<!-- if the slot is not specified, it should display the Card.svelte fallback slot -->
	<slot name="message" slot="message" {message}/>
</Card>

<!-- App.svelte -->
<!-- see question at SO: https://stackoverflow.com/q/77476736/47633 -->
<script>
	import Card from './Card.svelte'
	import ForwardCard from './ForwardCard.svelte'
</script>

<Card message='Card with fallback slot!' /> <!-- slot with fallback content: OK! -->

<Card message='Card with custom slot'> <!-- slot with custom content: OK! -->
	<div slot="message" let:message>custom slot: {message}</div>
</Card>

<ForwardCard message='ForwardCard with fallback slot!' /> <!-- forwarded slot with fallback content: WRONG!!! -->

<div>ERROR: MISSING "fallback slot: ForwardCard with default fallback!"</div>

<hr />

<ForwardCard message='ForwardCard with custom slot' > <!-- forwarded slot with custom content: OK! -->
	<div slot="message" let:message>custom slot: {message}</div>
</ForwardCard>

output:

This is a Card.svelte
fallback slot: Card with fallback slot!

This is a Card.svelte
custom slot: Card with custom slot

This is a ForwardCard.svelte
ERROR: MISSING "fallback slot: ForwardCard with default fallback!"

This is a ForwardCard.svelte
custom slot: ForwardCard with custom slot

Note: I also tried with Svelte 5 snippets and it works ok (great!!!)

svelte 5 repl: link

---

note: it seems to me like this line <slot name="message" slot="message" {message}/> is sending an empty slot, instead of no slot at all, and that's why the fallback is not being called. perhaps it's not a bug, but it's just that I couldn't find a way to forward no slot.

In other words, When ForwardCard doesn't receive any message slot, it shouldn't send any message slot to Card, instead of sending an empty slot, like I suspect it's doing right now.

Reproduction

svelte 4 repl with error

svelte 5 repl working OK

Logs

No response

System Info

repl online version version=4.2.3

Severity

annoyance

[opensas]

additional info: related question at stackoverflow

[dummdidumm]

There are many such edge cases and small gotchas with the current slot system. Closing because snippets replace slots with a more predictable and flexible solution.