From f8747b9774ef02422cc30ecbb08705067c6f120c Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Fri, 5 Sep 2025 17:02:19 +0200
Subject: [PATCH] detect async_deriveds inside batches that are destroyed in
 later batches

---
 .../src/internal/client/reactivity/batch.js   | 42 +++++++++++++++++++
 .../internal/client/reactivity/deriveds.js    |  4 +-
 .../_config.js                                | 31 ++++++++++++++
 .../main.svelte                               | 15 +++++--
 .../main.svelte                               |  5 +++
 5 files changed, 92 insertions(+), 5 deletions(-)

diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index e647f478be..75cd75da8f 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -344,6 +344,48 @@ export class Batch {
 		current_batch = this;
 	}
 
+	/**
+	 * Check if the branch this effect is in is obsolete in a later batch.
+	 * That is, if the branch exists in this batch but will be destroyed in a later batch.
+	 * @param {Effect} effect
+	 */
+	branch_obsolete(effect) {
+		/** @type {Effect[]} */
+		let alive = [];
+		/** @type {Effect[]} */
+		let skipped = [];
+		/** @type {Effect | null} */
+		let current = effect;
+
+		while (current !== null) {
+			if ((current.f & (BRANCH_EFFECT | ROOT_EFFECT)) !== 0) {
+				alive.push(current);
+				if (this.skipped_effects.has(current)) {
+					skipped.push(...alive);
+					alive = [];
+				}
+			}
+			current = current.parent;
+		}
+
+		let check = false;
+		for (const b of batches) {
+			if (b === this) {
+				check = true;
+			} else if (check) {
+				if (
+					alive.some((branch) => b.skipped_effects.has(branch)) ||
+					// TODO do we even have to check skipped here? how would an async_derived run for a branch that was already skipped?
+					(skipped.length > 0 && !skipped.some((branch) => b.skipped_effects.has(branch)))
+				) {
+					return true;
+				}
+			}
+		}
+
+		return false;
+	}
+
 	deactivate() {
 		current_batch = null;
 		previous_batch = null;
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index b020b84723..96947ef0fc 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -115,7 +115,7 @@ export function async_derived(fn, location) {
 	// only suspend in async deriveds created on initialisation
 	var should_suspend = !active_reaction;
 
-	async_effect(() => {
+	var effect = async_effect(() => {
 		if (DEV) current_async_effect = active_effect;
 
 		try {
@@ -153,7 +153,7 @@ export function async_derived(fn, location) {
 			batch.activate();
 
 			if (error) {
-				if (error !== STALE_REACTION) {
+				if (error !== STALE_REACTION && !batch.branch_obsolete(effect)) {
 					signal.f |= ERROR_VALUE;
 
 					// @ts-expect-error the error is the wrong type, but we don't care
diff --git a/packages/svelte/tests/runtime-runes/samples/async-block-reject-each-during-init/_config.js b/packages/svelte/tests/runtime-runes/samples/async-block-reject-each-during-init/_config.js
index c5dae7fee2..18f59ce3c8 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-block-reject-each-during-init/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/async-block-reject-each-during-init/_config.js
@@ -8,9 +8,11 @@ export default test({
 		increment.click();
 		await tick();
 
+		reject.click();
 		reject.click();
 		await tick();
 
+		resolve.click();
 		resolve.click();
 		await tick();
 
@@ -22,6 +24,35 @@ export default test({
 				<button>reject</button>
 				<p>false</p>
 				<p>1</p>
+				<p>false</p>
+				<p>1</p>
+			`
+		);
+
+		increment.click();
+		await tick();
+
+		increment.click();
+		await tick();
+
+		reject.click();
+		reject.click();
+		await tick();
+
+		resolve.click();
+		resolve.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<button>resolve</button>
+				<button>reject</button>
+				<p>false</p>
+				<p>3</p>
+				<p>false</p>
+				<p>3</p>
 			`
 		);
 	}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-block-reject-each-during-init/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-block-reject-each-during-init/main.svelte
index 1ad6cb84de..f17f492292 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-block-reject-each-during-init/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/async-block-reject-each-during-init/main.svelte
@@ -3,7 +3,7 @@
 
 	let deferreds = [];
 
-	function push() {
+	function push(_just_so_that_template_is_reactive_) {
 		const deferred = Promise.withResolvers();
 		deferreds.push(deferred);
 		return deferred.promise;
@@ -17,10 +17,19 @@
 <svelte:boundary>
 	{#if count % 2 === 0}
 		<p>true</p>
-		{#each await push() as count}<p>{count}</p>{/each}
+		{#each await push(count) as count}<p>{count}</p>{/each}
 	{:else}
 		<p>false</p>
-		{#each await push() as count}<p>{count}</p>{/each}
+		{#each await push(count) as count}<p>{count}</p>{/each}
+	{/if}
+	
+	{#if count % 2 === 0}
+		<p>true</p>
+		{#each await push(count) as count}<p>{count}</p>{/each}
+	{/if}
+	{#if count % 2 === 1}
+		<p>false</p>
+		{#each await push(count) as count}<p>{count}</p>{/each}
 	{/if}
 
 	{#snippet pending()}
diff --git a/packages/svelte/tests/runtime-runes/samples/async-top-level-error-nested-obsolete/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-top-level-error-nested-obsolete/main.svelte
index 2f461e96c8..2d8032228a 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-top-level-error-nested-obsolete/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/async-top-level-error-nested-obsolete/main.svelte
@@ -3,6 +3,11 @@
 	export let route = $state({ current: 'home' });
 </script>
 
+<script>
+	// reset from earlier tests
+	route.current = 'home'
+</script>
+
 <button onclick={() => route.reject()}>reject</button>
 
 <svelte:boundary>