Async Svelte crash in certain situations when destructuring awaited derived value in a @const

[dimfeld]

Describe the bug

This seems to happen when you have:

First, an awaited query, and a derived expression from that query

import { getIt } from './query.remote.ts';
let d = $derived(await getIt());
// For some reason this additional derived is required to reproduce
let showFetchCta = $derived(d.hasData)

In the template, a @const statement that destructures the awaited value.

{#if d}
	{@const {data, hasData} = d}

	{#if hasData}
		<p>The Data: {data}</p>
	{:else if showFetchCta} <!-- reference the derived so it doesn't get thrown away -->
		<p>Fetch now</p>
	{:else}
		<p>No data</p>
	{/if}
{/if}

Causes the error

 TypeError: Cannot read properties of undefined (reading 'f')

In Module.get where it reads signal.f (signal is apparently still undefined).

If you remove the @const destructuring and just reference the values as d.data and so on, then it works fine.

Reproduction

https://www.sveltelab.dev/38gut2mu4bw9p4d

Note you might have to run with the inspector open to really see the crash. Otherwise it's not totally obvious all the time.

Logs

Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'f')

	in <unknown>
	in +page.svelte
	in +layout.svelte
	in root.svelte
	in undefined

    at Module.get (chunk-O3RU4Q5E.js?v=4ba18bce:4157:23)
    at +page.svelte?t=1774689309840:99:13
    at chunk-GMPPHB2E.js?v=4ba18bce:798:5
    at update_reaction (chunk-O3RU4Q5E.js?v=4ba18bce:3983:18)
    at update_effect (chunk-O3RU4Q5E.js?v=4ba18bce:4122:21)
    at create_effect (chunk-O3RU4Q5E.js?v=4ba18bce:3466:7)
    at block (chunk-O3RU4Q5E.js?v=4ba18bce:3632:17)
    at Module.if_block (chunk-GMPPHB2E.js?v=4ba18bce:796:3)
    at +page.svelte?t=1774689309840:98:16
    at Module.add_svelte_meta (chunk-O3RU4Q5E.js?v=4ba18bce:600:12)

System Info

SvelteLab, also reproduced locally with latest Svelte and SvelteKit

Severity

annoyance

[7nik]

REPL