proxify values assigned to private state fields

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

@@ -39,18 +39,21 @@ function build_assignment(operator, left, right, context) {
 
 			if (private_state !== undefined) {
 				let transformed = false;
-				let value = /** @type {Expression} */ (
-					context.visit(build_assignment_value(operator, left, right))
-				);
 
-				if (should_proxy(value, context.state.scope)) {
+				if (
+					private_state.kind === 'state' &&
+					// other operators result in coercion
+					['=', '||=', '&&=', '??='].includes(operator) &&
+					should_proxy(right, context.state.scope)
+				) {
 					transformed = true;
-					value =
-						private_state.kind === 'raw_state'
-							? value
-							: build_proxy_reassignment(value, b.member(b.this, private_state.id));
+					right = build_proxy_reassignment(right, b.member(b.this, private_state.id));
 				}
 
+				let value = /** @type {Expression} */ (
+					context.visit(build_assignment_value(operator, left, right))
+				);
+
 				if (!context.state.in_constructor) {
 					return b.call('$.set', left, value);
 				} else if (transformed) {

packages/svelte/tests/runtime-runes/samples/proxy-nullish-coalescing-assignment/_config.js

@@ -2,15 +2,24 @@ import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
-	html: `<button>items: null</button>`,
+	html: `<button>items: null</button><button>items: null</button>`,
 
 	test({ assert, target }) {
-		const btn = target.querySelector('button');
+		const [btn1, btn2] = target.querySelectorAll('button');
 
-		flushSync(() => btn?.click());
-		assert.htmlEqual(target.innerHTML, `<button>items: [0]</button>`);
+		flushSync(() => btn1.click());
+		assert.htmlEqual(target.innerHTML, `<button>items: [0]</button><button>items: null</button>`);
 
-		flushSync(() => btn?.click());
-		assert.htmlEqual(target.innerHTML, `<button>items: [0,1]</button>`);
+		flushSync(() => btn1.click());
+		assert.htmlEqual(target.innerHTML, `<button>items: [0,1]</button><button>items: null</button>`);
+
+		flushSync(() => btn2.click());
+		assert.htmlEqual(target.innerHTML, `<button>items: [0,1]</button><button>items: [0]</button>`);
+
+		flushSync(() => btn2.click());
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>items: [0,1]</button><button>items: [0,1]</button>`
+		);
 	}
 });

packages/svelte/tests/runtime-runes/samples/proxy-nullish-coalescing-assignment/main.svelte

@@ -1,7 +1,25 @@
 <script>
 	let items = $state(null);
+
+	class Foo {
+		#items = $state(null);
+
+		get items() {
+			return this.#items;
+		}
+
+		add() {
+			(this.#items ??= []).push(this.#items.length);
+		}
+	}
+
+	const foo = new Foo();
 </script>
 
 <button onclick={() => (items ??= []).push(items.length)}>
 	items: {JSON.stringify(items)}
 </button>
+
+<button onclick={() => foo.add()}>
+	items: {JSON.stringify(foo.items)}
+</button>