If blocks in nested slots break the app

[christianheine]

Description
When using an if-block directly in a nested slot, it causes the app to break.

To Reproduce

• Open REPL: https://svelte.dev/repl/b78f73d066784716806a64d86a0ed58a?version=3.20.1
• Simply open the console and click the checkbox.

Stack trace

VM260:541 Uncaught (in promise) ReferenceError: span is not defined
at Object.update [as p] (eval at handle_message (VM254 about:srcdoc:13), :541:21)
at Object.update [as p] (eval at handle_message (VM254 about:srcdoc:13), :597:35)
at Object.update [as p] (eval at handle_message (VM254 about:srcdoc:13), :428:21)
at update (eval at handle_message (VM254 about:srcdoc:13), :167:40)
at flush (eval at handle_message (VM254 about:srcdoc:13), :136:17)

Information about your Svelte project:

• Your browser and the version: Chrome (latest)
• OS X

Severity
Hard to judge. On the one hand, it breaks the app. But on the other hand, there is a straightforward workaround, so it can be easily mitigated: As soon as the if-block is contained in another element it works: https://svelte.dev/repl/803509fd37d3464c879e2af530db7ba1?version=3.20.1

Will break:

<Inner>
	<span slot="label">
		<slot>
			Click on the checkbox to break the app
			{#if showMore}
				more
			{/if}
		</slot>
	</span>
</Inner>

Will work:

<Inner>
	<span slot="label">
		<slot>
			<span>
				This one will not break
				{#if showMore}
					more
				{/if}
			</span>
		</slot>
	</span>
</Inner>

[Conduitry]

Fixed in 3.21.0 - https://svelte.dev/repl/b78f73d066784716806a64d86a0ed58a?version=3.21.0