fix, changelog

CHANGELOG.md

@@ -11,6 +11,7 @@
 * **breaking** Overhaul and drastically improve creating custom elements with Svelte (see PR for list of changes and migration instructions) ([#8457](https://github.com/sveltejs/svelte/pull/8457))
 * **breaking** Deprecate `SvelteComponentTyped`, use `SvelteComponent` instead ([#8512](https://github.com/sveltejs/svelte/pull/8512))
 * **breaking** Error on falsy values instead of stores passed to `derived` ([#7947](https://github.com/sveltejs/svelte/pull/7947))
+* **breaking** Custom store implementers now need to pass an `update` function additionally to the `set` function ([#6750](https://github.com/sveltejs/svelte/pull/6750))
 * Add `a11y no-noninteractive-element-interactions` rule ([#8391](https://github.com/sveltejs/svelte/pull/8391))
 * Add `a11y-no-static-element-interactions`rule ([#8251](https://github.com/sveltejs/svelte/pull/8251))
 * Bind `null` option and input values consistently ([#8312](https://github.com/sveltejs/svelte/issues/8312))

src/runtime/store/index.ts

@@ -137,20 +137,6 @@ type StoresValues<T> = T extends Readable<infer U> ? U :
 	initial_value?: T
 ): Readable<T>;
 
-/**
- * Derived value store by synchronizing one or more readable stores and
- * applying an aggregation function over its input values.
- *
- * @param stores - input stores
- * @param fn - function callback that aggregates the values
- * @param initial_value - when used asynchronously
- */
-export function derived<S extends Stores, T>(
-	stores: S,
-	fn: (values: StoresValues<S>, set: Subscriber<T>) => Unsubscriber | void,
-	initial_value?: T
-): Readable<T>;
-
 /**
  * Derived value store by synchronizing one or more readable stores and
  * applying an aggregation function over its input values.
@@ -186,7 +172,7 @@ export function derived<T>(stores: Stores, fn: Function, initial_value?: T): Rea
 
 	const auto = fn.length < 2;
 
-	return readable(initial_value, (set) => {
+	return readable(initial_value, (set, update) => {
 		let started = false;
 		const values = [];