fix + test

packages/svelte/src/compiler/migrate/index.js

@@ -739,7 +739,11 @@ const instance_script = {
 			);
 			const bindings = ids.map((id) => state.scope.get(id.name));
 			const reassigned_bindings = bindings.filter((b) => b?.reassigned);
-			if (reassigned_bindings.length === 0 && !bindings.some((b) => b?.kind === 'store_sub') && node.body.expression.left.type === "Identifier") {
+			if (
+				reassigned_bindings.length === 0 &&
+				!bindings.some((b) => b?.kind === 'store_sub') &&
+				node.body.expression.left.type !== 'MemberExpression'
+			) {
 				let { start, end } = /** @type {{ start: number, end: number }} */ (
 					node.body.expression.right
 				);

packages/svelte/tests/migrate/samples/effects/output.svelte

@@ -1,7 +1,7 @@
 <script>
 	import { run } from 'svelte/legacy';
 
-	let count = 0;
+	let count = $state(0);
 	run(() => {
 		console.log(count);
 	});
@@ -18,6 +18,9 @@
 	run(() => {
 		$count = 1;
 	});
+	run(() => {
+		$count.x = count;
+	});
 </script>
 
-<button onclick={() => count++}>increment</button>
+<button onclick={() => count++}>increment</button>