fix: make template store subscriptions wait for the promise that assigns the store

.changeset/async-store-sub-blocker.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: block template store subscriptions on the promise that assigns the store

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -1265,6 +1265,13 @@ function calculate_blockers(instance, analysis) {
 
 		binding.blocker = /** @type {typeof binding['blocker']} */ (blocker);
 	}
+
+	// a store subscription must wait on whatever blocks the store itself
+	for (const [name, binding] of instance.scope.declarations) {
+		if (binding.kind === 'store_sub') {
+			binding.blocker ??= instance.scope.get(name.slice(1))?.blocker ?? null;
+		}
+	}
 }
 
 /**

packages/svelte/tests/runtime-runes/samples/async-store-sub-blocker/_config.js

@@ -0,0 +1,13 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// Tests that a store subscription only present in the template waits for the
+// promise that assigns the store instead of subscribing to `undefined`.
+export default test({
+	mode: ['client', 'hydrate', 'async-server'],
+	ssrHtml: '<p>hello</p>',
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>hello</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-store-sub-blocker/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import { writable } from 'svelte/store';
+
+	async function get_store() {
+		return writable('hello');
+	}
+
+	const store = await get_store();
+</script>
+
+<p>{$store}</p>