fix: avoid waterfall-warning when async resolves to same value (#18297)

.changeset/swift-terms-exist.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid waterfall-warning when async resolves to same value

packages/svelte/src/internal/client/reactivity/deriveds.js

@@ -230,9 +230,7 @@ export function async_derived(fn, label, location) {
 					signal.f ^= ERROR_VALUE;
 				}
 
-				internal_set(signal, value);
-
-				if (DEV && location !== undefined) {
+				if (DEV && location !== undefined && !signal.equals(value)) {
 					recent_async_deriveds.add(signal);
 
 					setTimeout(() => {
@@ -242,6 +240,8 @@ export function async_derived(fn, label, location) {
 						}
 					});
 				}
+
+				internal_set(signal, value);
 			}
 
 			batch.deactivate();

packages/svelte/tests/runtime-runes/samples/async-derived-same-value/_config.js

@@ -0,0 +1,19 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: { dev: true },
+	async test({ assert, target, warnings }) {
+		await tick();
+		const [button] = target.querySelectorAll('button');
+
+		button.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<button>1</button>');
+
+		button.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<button>1</button>');
+		assert.deepEqual(warnings, []);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-derived-same-value/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	let count = $state(0);
+	let unreactive = $derived(await push(count));
+
+	function push(v) {
+		if (!v) return v;
+
+		return Promise.resolve(1);
+	}
+</script>
+
+<button onclick={() => count++}>{unreactive}</button>