fix: re-evaluate derived props during teardown

.changeset/light-rivers-jump.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+Store deriveds old value before updating them for consistency with directly assigned sources when reading in teardown functions

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

@@ -382,7 +382,6 @@ export function prop(props, key, flags, fallback) {
 			return value;
 		}
 
-		// TODO is this still necessary post-#16263?
 		if (has_destroyed_component_ctx(d)) {
 			return d.v;
 		}

packages/svelte/src/internal/client/runtime.js

@@ -26,7 +26,7 @@ import {
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
 import { internal_set, old_values } from './reactivity/sources.js';
-import { destroy_derived_effects, update_derived } from './reactivity/deriveds.js';
+import { destroy_derived_effects, execute_derived, update_derived } from './reactivity/deriveds.js';
 import * as e from './errors.js';
 
 import { tracing_mode_flag } from '../flags/index.js';
@@ -769,7 +769,7 @@ export function get(signal) {
 		}
 	}
 
-	if (is_derived) {
+	if (is_derived && !is_destroying_effect) {
 		derived = /** @type {Derived} */ (signal);
 
 		if (check_dirtiness(derived)) {
@@ -810,8 +810,19 @@ export function get(signal) {
 		}
 	}
 
-	if (is_destroying_effect && old_values.has(signal)) {
-		return old_values.get(signal);
+	if (is_destroying_effect) {
+		if (old_values.has(signal)) {
+			return old_values.get(signal);
+		}
+
+		if (is_derived) {
+			derived = /** @type {Derived} */ (signal);
+
+			var value = (derived.f & CLEAN) !== 0 ? execute_derived(derived) : derived.v;
+			old_values.set(derived, value);
+
+			return value;
+		}
 	}
 
 	return signal.v;

packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/_config.js

@@ -0,0 +1,22 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, logs, target }) {
+		const [increment] = target.querySelectorAll('button');
+
+		flushSync(() => increment.click());
+		flushSync(() => increment.click());
+		flushSync(() => increment.click());
+
+		assert.deepEqual(logs, [
+			'count: 1',
+			'squared: 1',
+			'count: 2',
+			'squared: 4',
+			'count: 3',
+			'squared: 9',
+			'count: 4'
+		]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/derived-cleanup-old-value/main.svelte

@@ -0,0 +1,20 @@
+<script>
+	let count = $state(1);
+	let squared = $derived(count * count);
+
+	$effect(() => {
+		console.log(`count: ${count}`);
+
+		return () => {
+			console.log(`squared: ${squared}`);
+		};
+	});
+</script>
+
+<button onclick={() => count++}>increment</button>
+
+<p>count: {count}</p>
+
+{#if count % 2 === 0}
+	<p id="squared">squared: {squared}</p>
+{/if}