From 5c0a4a02a5f4429a2687d9d4c627a4638b332d88 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Wed, 25 Jun 2025 16:51:40 -0400
Subject: [PATCH] for unowned deriveds, throw errors lazily

---
 .../svelte/src/internal/client/error-handling.js   | 14 ++++++++++----
 packages/svelte/src/internal/client/runtime.js     |  6 +++++-
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/packages/svelte/src/internal/client/error-handling.js b/packages/svelte/src/internal/client/error-handling.js
index 594ef72d25..b88f99041b 100644
--- a/packages/svelte/src/internal/client/error-handling.js
+++ b/packages/svelte/src/internal/client/error-handling.js
@@ -1,17 +1,23 @@
-/** @import { Effect } from '#client' */
+/** @import { Derived, Effect } from '#client' */
 /** @import { Boundary } from './dom/blocks/boundary.js' */
 import { DEV } from 'esm-env';
 import { FILENAME } from '../../constants.js';
 import { is_firefox } from './dom/operations.js';
-import { BOUNDARY_EFFECT, EFFECT_RAN } from './constants.js';
+import { ASYNC_ERROR, BOUNDARY_EFFECT, EFFECT_RAN } from './constants.js';
 import { define_property, get_descriptor } from '../shared/utils.js';
-import { active_effect } from './runtime.js';
+import { active_effect, active_reaction } from './runtime.js';
 
 /**
  * @param {unknown} error
  */
 export function handle_error(error) {
-	var effect = /** @type {Effect} */ (active_effect);
+	var effect = active_effect;
+
+	// for unowned deriveds, don't throw until we read the value
+	if (effect === null) {
+		/** @type {Derived} */ (active_reaction).f |= ASYNC_ERROR;
+		return error;
+	}
 
 	if (DEV && error instanceof Error) {
 		// adjust_error(error, effect);
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index ae3cc36c34..b1a59de3d6 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -369,9 +369,13 @@ export function update_reaction(reaction) {
 			}
 		}
 
+		if ((reaction.f & ASYNC_ERROR) !== 0) {
+			reaction.f ^= ASYNC_ERROR;
+		}
+
 		return result;
 	} catch (error) {
-		handle_error(error);
+		return handle_error(error);
 	} finally {
 		reaction.f ^= REACTION_IS_UPDATING;
 		new_deps = previous_deps;