fix: increment private state fields through a non-`this` receiver (#18622)

.changeset/quiet-donkeys-clap.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: increment private state fields through a non-`this` receiver

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

@@ -13,7 +13,6 @@ export function UpdateExpression(node, context) {
 
 	if (
 		argument.type === 'MemberExpression' &&
-		argument.object.type === 'ThisExpression' &&
 		argument.property.type === 'PrivateIdentifier' &&
 		context.state.state_fields.has('#' + argument.property.name)
 	) {

packages/svelte/tests/runtime-runes/samples/class-private-state-increment-other-receiver/_config.js

@@ -0,0 +1,22 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>1</button><button>drop</button>`,
+
+	test({ assert, target }) {
+		const [bump, drop] = target.querySelectorAll('button');
+
+		bump?.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, `<button>2</button><button>drop</button>`);
+
+		bump?.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, `<button>3</button><button>drop</button>`);
+
+		drop?.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, `<button>2</button><button>drop</button>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/class-private-state-increment-other-receiver/main.svelte

@@ -0,0 +1,23 @@
+<script>
+	class Counter {
+		#count = $state(1);
+
+		bump(other) {
+			other.#count++;
+		}
+
+		drop(other) {
+			--other.#count;
+		}
+
+		getCount() {
+			return this.#count;
+		}
+	}
+
+	const a = new Counter();
+	const b = new Counter();
+</script>
+
+<button onclick={() => a.bump(b)}>{b.getCount()}</button>
+<button onclick={() => a.drop(b)}>drop</button>