fix: error on bind:this to each block parameter (#12638)

* chore: remove outdated comment

* fix: error on `bind:this` to each block parameter

Author: Rich-Harris

.changeset/sharp-spies-live.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: error on `bind:this` to each block parameter

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

@@ -385,33 +385,31 @@ const validation = {
 
 		const binding = context.state.scope.get(left.name);
 
-		if (
-			assignee.type === 'Identifier' &&
-			node.name !== 'this' // bind:this also works for regular variables
-		) {
+		if (assignee.type === 'Identifier') {
 			// reassignment
 			if (
-				!binding ||
-				(binding.kind !== 'state' &&
-					binding.kind !== 'frozen_state' &&
-					binding.kind !== 'prop' &&
-					binding.kind !== 'bindable_prop' &&
-					binding.kind !== 'each' &&
-					binding.kind !== 'store_sub' &&
-					!binding.mutated)
+				node.name !== 'this' && // bind:this also works for regular variables
+				(!binding ||
+					(binding.kind !== 'state' &&
+						binding.kind !== 'frozen_state' &&
+						binding.kind !== 'prop' &&
+						binding.kind !== 'bindable_prop' &&
+						binding.kind !== 'each' &&
+						binding.kind !== 'store_sub' &&
+						!binding.mutated))
 			) {
 				e.bind_invalid_value(node.expression);
 			}
 
-			if (binding.kind === 'derived') {
+			if (binding?.kind === 'derived') {
 				e.constant_binding(node.expression, 'derived state');
 			}
 
-			if (context.state.analysis.runes && binding.kind === 'each') {
+			if (context.state.analysis.runes && binding?.kind === 'each') {
 				e.each_item_invalid_assignment(node);
 			}
 
-			if (binding.kind === 'snippet') {
+			if (binding?.kind === 'snippet') {
 				e.snippet_parameter_assignment(node);
 			}
 		}

packages/svelte/tests/compiler-errors/samples/runes-invalid-each-binding-this/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'each_item_invalid_assignment',
+		message:
+			'Cannot reassign or bind to each block argument in runes mode. Use the array and index variables instead (e.g. `array[i] = value` instead of `entry = value`)'
+	}
+});

packages/svelte/tests/compiler-errors/samples/runes-invalid-each-binding-this/main.svelte

@@ -0,0 +1,11 @@
+<script lang="ts">
+	let array: Array<{ id: number; element: HTMLElement | null }> = $state([
+		{ id: 1, element: null },
+		{ id: 2, element: null },
+		{ id: 3, element: null }
+	]);
+</script>
+
+{#each array as { id, element } (id)}
+	<input bind:this={element} />
+{/each}