fix: add UNKNOWN evaluation value before breaking for binding.initial===SnippetBlock (#16910)

Author: paoloricciuti

.changeset/tall-parents-go.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: add `UNKNOWN` evaluation value before breaking for `binding.initial===SnippetBlock`

packages/svelte/src/compiler/phases/scope.js

@@ -263,6 +263,7 @@ class Evaluation {
 					if (binding.initial?.type === 'SnippetBlock') {
 						this.is_defined = true;
 						this.is_known = false;
+						this.values.add(UNKNOWN);
 						break;
 					}
 

packages/svelte/tests/runtime-runes/samples/const-snippet-reactive/Component.svelte

@@ -0,0 +1,5 @@
+<script>
+	let { test } = $props();
+</script>
+
+{@render test?.()}

packages/svelte/tests/runtime-runes/samples/const-snippet-reactive/_config.js

@@ -0,0 +1,10 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const btn = target.querySelector('button');
+		flushSync(() => btn?.click());
+		assert.htmlEqual(target.innerHTML, `<button></button><p>snip</p>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/const-snippet-reactive/main.svelte

@@ -0,0 +1,15 @@
+<script>
+	import Component from "./Component.svelte";
+	let count = $state(0);
+</script>
+
+{#snippet snip()}
+<p>snip</p>
+{/snippet}
+
+<button onclick={() => count++}></button>
+{#if true}
+	{@const test =  count % 2 === 0 ? undefined: snip}
+	<Component {test} />
+{/if}
+