From 135f4439c300b4bb996782e90f9d610b3373d574 Mon Sep 17 00:00:00 2001
From: Yuichiro Yamashita <xydybaseball@gmail.com>
Date: Fri, 14 Aug 2026 14:49:51 +0900
Subject: [PATCH] fix: increment private state fields through a non-`this`
 receiver (#18622)

---
 .changeset/quiet-donkeys-clap.md              |  5 ++++
 .../client/visitors/UpdateExpression.js       |  1 -
 .../_config.js                                | 22 ++++++++++++++++++
 .../main.svelte                               | 23 +++++++++++++++++++
 4 files changed, 50 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/quiet-donkeys-clap.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/class-private-state-increment-other-receiver/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/class-private-state-increment-other-receiver/main.svelte

diff --git a/.changeset/quiet-donkeys-clap.md b/.changeset/quiet-donkeys-clap.md
new file mode 100644
index 0000000000..dc9df3b7e1
--- /dev/null
+++ b/.changeset/quiet-donkeys-clap.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: increment private state fields through a non-`this` receiver
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js
index 66952eb298..bc483daaed 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/UpdateExpression.js
+++ b/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)
 	) {
diff --git a/packages/svelte/tests/runtime-runes/samples/class-private-state-increment-other-receiver/_config.js b/packages/svelte/tests/runtime-runes/samples/class-private-state-increment-other-receiver/_config.js
new file mode 100644
index 0000000000..8f023b602a
--- /dev/null
+++ b/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>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/class-private-state-increment-other-receiver/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-private-state-increment-other-receiver/main.svelte
new file mode 100644
index 0000000000..67827aceee
--- /dev/null
+++ b/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>