fix: add back derived type overload

fixes #9866

Author: dummdidumm

.changeset/famous-falcons-melt.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: add back `derived` type overload

packages/svelte/src/store/index.js

@@ -123,12 +123,38 @@ function run_all(fns) {
 }
 
 /**
- * @template {import('./private').Stores} S
+ * Derived value store by synchronizing one or more readable stores and
+ * applying an aggregation function over its input values.
+ *
+ * https://svelte.dev/docs/svelte-store#derived
+ * @template {import('./private.js').Stores} S
+ * @template T
+ * @overload
+ * @param {S} stores
+ * @param {(values: import('./private.js').StoresValues<S>) => T} fn
+ * @param {T} [initial_value]
+ * @returns {import('./public.js').Readable<T>}
+ */
+/**
+ * Derived value store by synchronizing one or more readable stores and
+ * applying an aggregation function over its input values.
+ *
+ * https://svelte.dev/docs/svelte-store#derived
+ * @template {import('./private.js').Stores} S
+ * @template T
+ * @overload
+ * @param {S} stores
+ * @param {(values: import('./private.js').StoresValues<S>, set: (value: T) => void, update: (fn: import('./public.js').Updater<T>) => void) => import('./public.js').Unsubscriber | void} fn
+ * @param {T} [initial_value]
+ * @returns {import('./public.js').Readable<T>}
+ */
+/**
+ * @template {import('./private.js').Stores} S
  * @template T
  * @param {S} stores
  * @param {Function} fn
  * @param {T} [initial_value]
- * @returns {import('./public').Readable<T>}
+ * @returns {import('./public.js').Readable<T>}
  */
 export function derived(stores, fn, initial_value) {
 	const single = !Array.isArray(stores);

packages/svelte/tests/types/store.ts

@@ -0,0 +1,13 @@
+import { derived, writable } from 'svelte/store';
+
+const a = writable(false);
+derived(a, (aVal) => {
+	// @ts-expect-error
+	aVal === '';
+	return aVal === true;
+});
+derived([a], ([aVal]) => {
+	// @ts-expect-error
+	aVal === '';
+	return aVal === true;
+});

packages/svelte/types/index.d.ts

@@ -1983,6 +1983,11 @@ declare module 'svelte/store' {
 		| Readable<any>
 		| [Readable<any>, ...Array<Readable<any>>]
 		| Array<Readable<any>>;
+
+	/** One or more values from `Readable` stores. */
+	type StoresValues<T> = T extends Readable<infer U>
+		? U
+		: { [K in keyof T]: T[K] extends Readable<infer U> ? U : never };
 	/**
 	 * Creates a `Readable` store that allows reading by subscription.
 	 *
@@ -1999,8 +2004,20 @@ declare module 'svelte/store' {
 	 * @param value initial value
 	 * */
 	export function writable<T>(value?: T | undefined, start?: StartStopNotifier<T> | undefined): Writable<T>;
-
-	export function derived<S extends Stores, T>(stores: S, fn: Function, initial_value?: T | undefined): Readable<T>;
+	/**
+	 * Derived value store by synchronizing one or more readable stores and
+	 * applying an aggregation function over its input values.
+	 *
+	 * https://svelte.dev/docs/svelte-store#derived
+	 * */
+	export function derived<S extends Stores, T>(stores: S, fn: (values: StoresValues<S>) => T, initial_value?: T | undefined): Readable<T>;
+	/**
+	 * Derived value store by synchronizing one or more readable stores and
+	 * applying an aggregation function over its input values.
+	 *
+	 * https://svelte.dev/docs/svelte-store#derived
+	 * */
+	export function derived<S extends Stores, T>(stores: S, fn: (values: StoresValues<S>, set: (value: T) => void, update: (fn: Updater<T>) => void) => Unsubscriber | void, initial_value?: T | undefined): Readable<T>;
 	/**
 	 * Takes a store and returns a new one derived from the old one that is readable.
 	 *