fix: don't freeze non-POJOs

The main reason for Object.freeze on `$state.frozen` is to mark a boundary for `$state` - but since that one doesn't traverse non-POJOs either, we should leave class instances etc alone, too.
Since "this is a POJO and not a class instance" cannot be described in typescript, the `Readonly` modifier had to be removed from the return type.

.changeset/smart-readers-look.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: don't freeze non-POJOs

packages/svelte/src/ambient.d.ts

@@ -19,7 +19,7 @@ declare function $state<T>(): T | undefined;
 
 declare namespace $state {
 	/**
-	 * Declares reactive read-only state that is shallowly immutable.
+	 * Declares reactive read-only state that is shallowly immutable, if it's a plain object or array.
 	 *
 	 * Example:
 	 * ```ts
@@ -40,8 +40,8 @@ declare namespace $state {
 	 *
 	 * @param initial The initial value
 	 */
-	export function frozen<T>(initial: T): Readonly<T>;
-	export function frozen<T>(): Readonly<T> | undefined;
+	export function frozen<T>(initial: T): T;
+	export function frozen<T>(): T | undefined;
 }
 
 /**

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

@@ -1301,12 +1301,16 @@ if (DEV) {
  */
 export function freeze(value) {
 	if (typeof value === 'object' && value != null && !is_frozen(value)) {
-		// If the object is already proxified, then unstate the value
-		if (STATE_SYMBOL in value) {
-			return object_freeze(unstate(value));
+		const prototype = get_prototype_of(value);
+		// Leave class instances etc alone, they're not traversed by $state, either
+		if (prototype === object_prototype || prototype === array_prototype) {
+			// If the object is already proxified, then unstate the value
+			if (STATE_SYMBOL in value) {
+				return object_freeze(unstate(value));
+			}
+			// Otherwise freeze the object
+			object_freeze(value);
 		}
-		// Otherwise freeze the object
-		object_freeze(value);
 	}
 	return value;
 }

packages/svelte/tests/runtime-runes/samples/frozen-state-class/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `1`
+});

packages/svelte/tests/runtime-runes/samples/frozen-state-class/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	class Counter {
+		count = 0
+	}
+	const counter = $state.frozen(new Counter());
+	counter.count++;
+</script>
+
+{counter.count}

packages/svelte/types/index.d.ts

@@ -2528,7 +2528,7 @@ declare function $state<T>(): T | undefined;
 
 declare namespace $state {
 	/**
-	 * Declares reactive read-only state that is shallowly immutable.
+	 * Declares reactive read-only state that is shallowly immutable, if it's a plain object or array.
 	 *
 	 * Example:
 	 * ```ts
@@ -2549,8 +2549,8 @@ declare namespace $state {
 	 *
 	 * @param initial The initial value
 	 */
-	export function frozen<T>(initial: T): Readonly<T>;
-	export function frozen<T>(): Readonly<T> | undefined;
+	export function frozen<T>(initial: T): T;
+	export function frozen<T>(): T | undefined;
 }
 
 /**