fix: legacy reactive dependencies tweak (#10128)

take into account member expressions when determining legacy reactive dependencies
fixes #9954

.changeset/quiet-crabs-nail.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: take into account member expressions when determining legacy reactive dependencies

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -500,7 +500,15 @@ const legacy_scope_tweaker = {
 			node.body.type === 'ExpressionStatement' &&
 			node.body.expression.type === 'AssignmentExpression'
 		) {
-			for (const id of extract_identifiers(node.body.expression.left)) {
+			let ids = extract_identifiers(node.body.expression.left);
+			if (node.body.expression.left.type === 'MemberExpression') {
+				const id = object(node.body.expression.left);
+				if (id !== null) {
+					ids = [id];
+				}
+			}
+
+			for (const id of ids) {
 				const binding = state.scope.get(id.name);
 				if (binding?.kind === 'legacy_reactive') {
 					// TODO does this include `let double; $: double = x * 2`?
@@ -511,10 +519,17 @@ const legacy_scope_tweaker = {
 	},
 	AssignmentExpression(node, { state, next }) {
 		if (state.reactive_statement && node.operator === '=') {
+			if (node.left.type === 'MemberExpression') {
+				const id = object(node.left);
+				if (id !== null) {
+					state.reactive_statement.assignments.add(id);
+				}
+			} else {
 				for (const id of extract_identifiers(node.left)) {
 					state.reactive_statement.assignments.add(id);
 				}
 			}
+		}
 
 		next();
 	},

packages/svelte/tests/runtime-legacy/samples/reactive-value-assign-properties/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `1 1`
+});

packages/svelte/tests/runtime-legacy/samples/reactive-value-assign-properties/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	let button = { title: '', label: '' };
+	let title = '';
+	let label = '';
+
+	title = label = '1'; // to add dependencies/generate update block
+
+	$: button.title = title;
+	$: button.label = label;
+</script>
+
+{button.title} {button.label}