handle errors in async block expressions

packages/svelte/src/internal/client/dom/blocks/async.js

@@ -1,7 +1,7 @@
 /** @import { Effect, TemplateNode, Value } from '#client' */
 import { DESTROYED } from '#client/constants';
 import { async_derived } from '../../reactivity/deriveds.js';
-import { active_effect } from '../../runtime.js';
+import { active_effect, get } from '../../runtime.js';
 import { capture, get_pending_boundary } from './boundary.js';
 
 /**
@@ -20,12 +20,15 @@ export async function async(node, expressions, fn) {
 	boundary.update_pending_count(1);
 
 	try {
-		const result = await Promise.all(expressions.map((fn) => async_derived(fn)));
+		const deriveds = await Promise.all(expressions.map((fn) => async_derived(fn)));
+
+		// get deriveds eagerly to avoid creating blocks if they reject
+		for (const d of deriveds) get(d);
 
 		if ((parent.f & DESTROYED) !== 0) return;
 
 		restore();
-		fn(node, ...result);
+		fn(node, ...deriveds);
 	} catch (error) {
 		boundary.error(error);
 	} finally {

packages/svelte/tests/runtime-runes/samples/async-error-in-block-expression/_config.js

@@ -0,0 +1,11 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `loading`,
+
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, 'oops');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-error-in-block-expression/main.svelte

@@ -0,0 +1,8 @@
+<svelte:boundary>
+	{#each (await Promise.reject(new Error('oops'))) as x}
+		hi
+	{/each}
+
+	{#snippet pending()}loading{/snippet}
+	{#snippet failed()}oops{/snippet}
+</svelte:boundary>