fix: update expressions on server deriveds

.changeset/good-steaks-rhyme.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: update expressions on server deriveds

packages/svelte/src/compiler/phases/3-transform/server/visitors/UpdateExpression.js

@@ -9,17 +9,26 @@ import * as b from '#compiler/builders';
 export function UpdateExpression(node, context) {
 	const argument = node.argument;
 
-	if (
-		argument.type === 'Identifier' &&
-		context.state.scope.get(argument.name)?.kind === 'store_sub'
-	) {
-		return b.call(
-			node.prefix ? '$.update_store_pre' : '$.update_store',
-			b.assignment('??=', b.id('$$store_subs'), b.object([])),
-			b.literal(argument.name),
-			b.id(argument.name.slice(1)),
-			node.operator === '--' && b.literal(-1)
-		);
+	if (argument.type === 'Identifier') {
+		const binding = context.state.scope.get(argument.name);
+
+		if (binding?.kind === 'store_sub') {
+			return b.call(
+				node.prefix ? '$.update_store_pre' : '$.update_store',
+				b.assignment('??=', b.id('$$store_subs'), b.object([])),
+				b.literal(argument.name),
+				b.id(argument.name.slice(1)),
+				node.operator === '--' && b.literal(-1)
+			);
+		}
+
+		if (binding?.kind === 'derived') {
+			return b.call(
+				node.prefix ? '$.update_derived_pre' : '$.update_derived',
+				binding.node,
+				node.operator === '--' && b.literal(-1)
+			);
+		}
 	}
 
 	return context.next();

packages/svelte/src/internal/server/index.js

@@ -504,6 +504,35 @@ export function derived(fn) {
 	};
 }
 
+/**
+ * @template {number | bigint} T
+ * @param {(value?: T) => T} derived
+ * @param {1 | -1} [d]
+ * @returns {T}
+ */
+export function update_derived(derived, d = 1) {
+	const value = derived();
+	let increase = typeof value === 'bigint' ? BigInt(d) : d;
+	// for some reason TS is mad even if T is always number or bigint
+	derived(value + /** @type {*} */ (increase));
+	return value;
+}
+
+/**
+ * @template {number | bigint} T
+ * @param {(value?: T) => T} derived
+ * @param {1 | -1} [d]
+ * @returns {T}
+ */
+export function update_derived_pre(derived, d = 1) {
+	const old_value = derived();
+	let increase = typeof old_value === 'bigint' ? BigInt(d) : d;
+	// for some reason TS is mad even if T is always number or bigint
+	const value = old_value + /** @type {*} */ (increase);
+	derived(value);
+	return value;
+}
+
 /**
  * @template T
  * @param {()=>T} fn

packages/svelte/tests/runtime-runes/samples/derived-update-server/_config.js

@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+export default test({
+	mode: ['async-server', 'server'],
+	html: '<p>postfix: 0, postfix_minus: 1, prefix: 1, prefix_minus: 0, count: 0</p><p>postfix_n: 0, postfix_minus_n: 1, prefix_n: 1, prefix_minus_n: 0, count_n: 0</p>'
+});

packages/svelte/tests/runtime-runes/samples/derived-update-server/main.svelte

@@ -0,0 +1,17 @@
+<script>
+	let count = $derived(0);
+	let postfix = count++;
+	let postfix_minus = count--;
+	let prefix = ++count;
+	let prefix_minus = --count;
+
+	let count_n = $derived(0n);
+	let postfix_n = count_n++;
+	let postfix_minus_n = count_n--;
+	let prefix_n = ++count_n;
+	let prefix_minus_n = --count_n;
+</script>
+
+<p>postfix: {postfix}, postfix_minus: {postfix_minus}, prefix: {prefix}, prefix_minus: {prefix_minus}, count: {count}</p>
+<p>postfix_n: {postfix_n}, postfix_minus_n: {postfix_minus_n}, prefix_n: {prefix_n}, prefix_minus_n: {prefix_minus_n}, count_n: {count_n}</p>
+