fix when LHS of a reactive assignment is a member expression (#4212)

Author: Conduitry

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 * Prevent text input cursor jumping in Safari with one-way binding ([#3449](https://github.com/sveltejs/svelte/issues/3449))
 * Expose compiler version in dev events ([#4047](https://github.com/sveltejs/svelte/issues/4047))
+* Do not automatically declare variables in reactive declarations when assigning to a member expression ([#4212](https://github.com/sveltejs/svelte/issues/4212))
 
 ## 3.16.7
 

src/compiler/compile/Component.ts

@@ -603,6 +603,7 @@ export default class Component {
 
 			const { expression } = node.body;
 			if (expression.type !== 'AssignmentExpression') return;
+			if (expression.left.type === 'MemberExpression') return;
 
 			extract_names(expression.left).forEach(name => {
 				if (!this.var_lookup.has(name) && name[0] !== '$') {

test/runtime/samples/reactive-values-no-implicit-member-expression/_config.js

@@ -0,0 +1,5 @@
+export default {
+	test({ assert, window }) {
+		assert.equal(window.document.title, 'foo');
+	}
+};

test/runtime/samples/reactive-values-no-implicit-member-expression/main.svelte

@@ -0,0 +1,3 @@
+<script>
+	$: document.title = 'foo';
+</script>