fix: do not proxify the value assigned to a derived (#16302)

.changeset/new-wolves-punch.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: do not proxify the value assigned to a derived

packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js

@@ -137,6 +137,7 @@ function build_assignment(operator, left, right, context) {
 				binding.kind !== 'prop' &&
 				binding.kind !== 'bindable_prop' &&
 				binding.kind !== 'raw_state' &&
+				binding.kind !== 'derived' &&
 				binding.kind !== 'store_sub' &&
 				context.state.analysis.runes &&
 				should_proxy(right, context.state.scope) &&

packages/svelte/tests/runtime-runes/samples/writable-derived-4/_config.js

@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const [btn1, btn2, btn3] = target.getElementsByTagName('button');
+		const [div] = target.getElementsByTagName('div');
+
+		flushSync(() => btn1.click());
+		assert.equal(div.textContent, '1');
+		flushSync(() => btn2.click());
+		assert.equal(div.textContent, '1');
+		flushSync(() => btn3.click());
+		assert.equal(div.textContent, '2');
+
+		flushSync(() => btn1.click());
+		assert.equal(div.textContent, '2');
+		flushSync(() => btn2.click());
+		assert.equal(div.textContent, '2');
+		flushSync(() => btn3.click());
+		assert.equal(div.textContent, '3');
+	}
+});

packages/svelte/tests/runtime-runes/samples/writable-derived-4/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	let count = $derived({ value: 1 });
+</script>
+
+<button onclick={() => count.value++}>mutate</button>
+<button onclick={() => count = count}>assign self</button>
+<button onclick={() => count = {...count} }>assign copy</button>
+
+<div>{count.value}</div>