throw if argument is a state proxy

Author: Rich-Harris

packages/svelte/messages/client-errors/errors.md

@@ -60,6 +60,10 @@
 
 > The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files
 
+## state_frozen_invalid_argument
+
+> The argument to `$state.frozen(...)` cannot be an object created with `$state(...)`. You should create a copy of it first, for example with `$state.snapshot`
+
 ## state_prototype_fixed
 
 > Cannot set prototype of `$state` object

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

@@ -308,4 +308,20 @@ export function svelte_component_invalid_this_value() {
 		// TODO print a link to the documentation
 		throw new Error("svelte_component_invalid_this_value");
 	}
+}
+
+/**
+ * The argument to `$state.frozen(...)` cannot be an object created with `$state(...)`. You should create a copy of it first, for example with `$state.snapshot`
+ * @returns {never}
+ */
+export function state_frozen_invalid_argument() {
+	if (DEV) {
+		const error = new Error(`state_frozen_invalid_argument\nThe argument to \`$state.frozen(...)\` cannot be an object created with \`$state(...)\`. You should create a copy of it first, for example with \`$state.snapshot\``);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		// TODO print a link to the documentation
+		throw new Error("state_frozen_invalid_argument");
+	}
 }

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

@@ -1,6 +1,7 @@
 import { DEV } from 'esm-env';
 import { define_property, is_array, is_frozen, object_freeze } from '../shared/utils.js';
 import { STATE_FROZEN_SYMBOL, STATE_SYMBOL } from './constants.js';
+import * as e from './errors.js';
 
 /**
  * Expects a value that was wrapped with `freeze` and makes it frozen in DEV.
@@ -17,9 +18,8 @@ export function freeze(value) {
 	) {
 		var copy = /** @type {T} */ (value);
 
-		// If the object is already proxified, then clone the value
 		if (STATE_SYMBOL in value) {
-			copy = /** @type {T} */ (is_array(value) ? [...value] : { ...value });
+			e.state_frozen_invalid_argument();
 		}
 
 		define_property(copy, STATE_FROZEN_SYMBOL, {

packages/svelte/src/internal/client/freeze.test.ts

@@ -11,9 +11,6 @@ test('freezes an object', () => {
 	}, /Cannot assign to read only property/);
 });
 
-test('preserves classes', () => {
-	class Foo {}
-	const frozen = freeze(proxy([new Foo()]));
-
-	assert.ok(frozen[0] instanceof Foo);
+test('throws if argument is a state proxy', () => {
+	assert.throws(() => freeze(proxy({})), /state_frozen_invalid_argument/);
 });