fix: rethrow errors from await block if no catch block exists (#13819)

* chore: highlight swallowed errors from await blocks in DEV

* chore: highlight swallowed errors from await blocks in DEV

* chore: highlight swallowed errors from await blocks in DEV

* lint

* feedback

* feedback

* add test

* Update packages/svelte/tests/runtime-runes/samples/await-no-catch-error/main.svelte

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/blue-boats-call.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: rethrow errors from await block if no catch block exists

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

@@ -131,6 +131,10 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 					// but let's use internal_set for consistency and just to be safe
 					internal_set(error_source, error);
 					update(CATCH, true);
+					if (!catch_fn) {
+						// Rethrow the error if no catch block exists
+						throw error_source.v;
+					}
 				}
 			);
 

packages/svelte/tests/runtime-runes/samples/await-no-catch-error/_config.js

@@ -0,0 +1,22 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const b1 = target.querySelector('button');
+
+		let err = '';
+		window.addEventListener('error', (e) => {
+			e.preventDefault();
+			err = e.message;
+		});
+
+		b1?.click();
+		await Promise.resolve();
+		flushSync();
+
+		assert.throws(() => {
+			throw err;
+		}, /Test/);
+	}
+});

packages/svelte/tests/runtime-runes/samples/await-no-catch-error/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	const promise = Promise.reject('Test');
+	let toggle = $state(false);
+</script>
+
+<button onclick={() => toggle = !toggle}>toggle</button>
+
+{#if toggle}
+	{#await promise}{/await}
+{/if}