fix: ensure SvelteDate cached methods have correct reactive context

.changeset/wicked-zebras-exist.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: use correct reaction when lazily creating deriveds inside `SvelteDate`

packages/svelte/src/reactivity/date.js

@@ -1,8 +1,7 @@
 /** @import { Source } from '#client' */
-import { DESTROYED } from '../internal/client/constants.js';
 import { derived } from '../internal/client/index.js';
 import { source, set } from '../internal/client/reactivity/sources.js';
-import { get } from '../internal/client/runtime.js';
+import { active_reaction, get, set_active_reaction } from '../internal/client/runtime.js';
 
 var inited = false;
 
@@ -12,6 +11,8 @@ export class SvelteDate extends Date {
 	/** @type {Map<keyof Date, Source<unknown>>} */
 	#deriveds = new Map();
 
+	#reaction = active_reaction;
+
 	/** @param {any[]} params */
 	constructor(...params) {
 		// @ts-ignore
@@ -43,14 +44,21 @@ export class SvelteDate extends Date {
 
 					var d = this.#deriveds.get(method);
 
-					if (d === undefined || (d.f & DESTROYED) !== 0) {
+					if (d === undefined) {
+						// lazily create the derived, but as though it were being
+						// created at the same time as the class instance
+						const reaction = active_reaction;
+						set_active_reaction(this.#reaction);
+
 						d = derived(() => {
 							get(this.#time);
 							// @ts-ignore
 							return date_proto[method].apply(this, args);
 						});
 
 						this.#deriveds.set(method, d);
+
+						set_active_reaction(reaction);
 					}
 
 					return get(d);

packages/svelte/src/reactivity/date.test.ts

@@ -642,3 +642,33 @@ test('Date methods invoked for the first time in a derived', () => {
 
 	cleanup();
 });
+
+test('Date methods shared between deriveds', () => {
+	const date = new SvelteDate(initial_date);
+	const log: any = [];
+
+	const cleanup = effect_root(() => {
+		const year = derived(() => {
+			return date.getFullYear();
+		});
+		const year2 = derived(() => {
+			return date.getTime(), date.getFullYear();
+		});
+
+		render_effect(() => {
+			log.push(get(year) + '/' + get(year2).toString());
+		});
+
+		flushSync(() => {
+			date.setFullYear(date.getFullYear() + 1);
+		});
+
+		flushSync(() => {
+			date.setFullYear(date.getFullYear() + 1);
+		});
+	});
+
+	assert.deepEqual(log, ['2023/2023', '2024/2024', '2025/2025']);
+
+	cleanup();
+});