fix #5427 - do not declare variable for member assignment in reactive…

… declaration

src/compiler/compile/render_ssr/index.ts

@@ -99,8 +99,10 @@ export default function ssr(
 					? b`
 						${injected.map(name => b`let ${name};`)}
 						${statement}`
-					: b`
-						let ${left} = ${right}`;
+					: left.type === "Identifier"
+					? b`
+						let ${left} = ${right}`
+					: b`${left} = ${right}`;
 			}
 		} else { // TODO do not add label if it's not referenced
 			statement = b`$: { ${statement} }`;

test/runtime/samples/reactive-value-assign-property/_config.js

@@ -0,0 +1,5 @@
+export default {
+	html: `
+		<h1>Hello world!</h1>
+	`,
+};

test/runtime/samples/reactive-value-assign-property/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	$: user = {};
+	$: user.name = 'world';
+</script>
+
+<h1>Hello {user.name}!</h1>