Undefined values when using a let binding from the default slot inside a named slot.

[dimfeld]

Describe the bug

When using a value from a let:binding on the default slot inside a named slot, the bound value is sometimes undefined if the named slot is instantiated more than once in the child component.

<!-- App.svelte -->
<AThing let:showA>
	<div slot="a">
		{showA}
	</div>
</AThing>

<!-- AThing.svelte -->
<script>
	export let showA = true;
</script>

<slot name="a" />
<slot name="a" />

<slot {showA} />

I'm actually not sure if this is even valid to do. My intuition is that using a let binding from the default slot inside a named slot is sort of a scope violation and so the compiler should throw an error (or at least a warning to avoid breaking behavior).

If Svelte intentionally allows this, then the bug is just that showA is undefined in the first instantiation of the named slot.

Also related, the second half of this comment

Reproduction

https://svelte.dev/repl/a9bb52762959444da8da018ab04f6c7b?version=3.47.0

Logs

No response

System Info

Svelte REPL, 3.47.0. Also reproduced on 3.46.6 and this has presumably been around for a while.

Severity

annoyance

[dimfeld]

Related: #7485 and #7245

[gtm-nayan]

I'm actually not sure if this is even valid to do. My intuition is that using a let binding from the default slot inside a named slot is sort of a scope violation and so the compiler should throw an error (or at least a warning to avoid breaking behavior).

This is now disallowed from v4.0 onwards