From 5969b0919c1152c2851261ad8df05630500c0728 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 31 Jan 2025 18:26:34 -0500
Subject: [PATCH] waterfall detection

---
 .../src/internal/client/reactivity/deriveds.js     | 14 ++++++++++++++
 packages/svelte/src/internal/client/runtime.js     |  3 +++
 2 files changed, 17 insertions(+)

diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index f8a8aaddac..bb6a86cc2a 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -22,6 +22,7 @@ import {
 } from '../runtime.js';
 import { equals, safe_equals } from './equality.js';
 import * as e from '../errors.js';
+import * as w from '../warnings.js';
 import { block, destroy_effect } from './effects.js';
 import { inspect_effects, internal_set, set_inspect_effects, source } from './sources.js';
 import { get_stack } from '../dev/tracing.js';
@@ -37,6 +38,8 @@ export function set_from_async_derived(v) {
 	from_async_derived = v;
 }
 
+export const recent_async_deriveds = new Set();
+
 /**
  * @template V
  * @param {() => V} fn
@@ -117,6 +120,17 @@ export function async_derived(fn) {
 				from_async_derived = null;
 
 				internal_set(value, v);
+
+				if (DEV) {
+					recent_async_deriveds.add(value);
+
+					setTimeout(() => {
+						if (recent_async_deriveds.has(value)) {
+							w.await_waterfall();
+							recent_async_deriveds.delete(value);
+						}
+					});
+				}
 			}
 		} catch (e) {
 			handle_error(e, parent, null, parent.ctx);
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 716374d69f..2990c0dd69 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -38,6 +38,7 @@ import {
 	destroy_derived_effects,
 	execute_derived,
 	from_async_derived,
+	recent_async_deriveds,
 	update_derived
 } from './reactivity/deriveds.js';
 import * as e from './errors.js';
@@ -1064,6 +1065,8 @@ export function get(signal) {
 				entry.read.push(get_stack('TracedAt'));
 			}
 		}
+
+		recent_async_deriveds.delete(signal);
 	}
 
 	return signal.v;