fix: migrate reactive statements with inner blocks (#13675)

.changeset/smooth-apes-shave.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: migrate reactive statements with inner blocks

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

@@ -575,7 +575,8 @@ const instance_script = {
 
 				const labeled_has_single_assignment =
 					labeled_statement?.body.type === 'BlockStatement' &&
-					labeled_statement.body.body.length === 1;
+					labeled_statement.body.body.length === 1 &&
+					labeled_statement.body.body[0].type === 'ExpressionStatement';
 
 				const is_expression_assignment =
 					labeled_statement?.body.type === 'ExpressionStatement' &&

packages/svelte/tests/migrate/samples/reactive-statements-inner-block/input.svelte

@@ -0,0 +1,17 @@
+<script lang="ts">
+	let menuElement: HTMLUListElement | undefined = undefined;
+	let left: number;
+  let top: number;
+
+  $: {
+    if (menuElement) {
+      const rect = menuElement.getBoundingClientRect();
+      const menuHeight = 0;
+
+      left = window.innerWidth - rect.width
+      top = window.innerHeight - menuHeight;
+    }
+  }
+</script>
+
+<ul bind:this={menuElement}></ul>

packages/svelte/tests/migrate/samples/reactive-statements-inner-block/output.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+  import { run } from 'svelte/legacy';
+
+	let menuElement: HTMLUListElement | undefined = $state(undefined);
+	let left: number = $state();
+  let top: number = $state();
+
+  run(() => {
+    if (menuElement) {
+      const rect = menuElement.getBoundingClientRect();
+      const menuHeight = 0;
+
+      left = window.innerWidth - rect.width
+      top = window.innerHeight - menuHeight;
+    }
+  });
+</script>
+
+<ul bind:this={menuElement}></ul>