fix: falsy attachments on components (#16021)

* fix: falsy attachments on components

* skip the noop if known to be a function

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/fuzzy-taxis-hunt.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: falsy attachments on components

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js

@@ -258,10 +258,18 @@ export function build_component(node, component_name, context, anchor = context.
 				}
 			}
 		} else if (attribute.type === 'AttachTag') {
+			const evaluated = context.state.scope.evaluate(attribute.expression);
+
 			let expression = /** @type {Expression} */ (context.visit(attribute.expression));
 
 			if (attribute.metadata.expression.has_state) {
-				expression = b.arrow([b.id('$$node')], b.call(expression, b.id('$$node')));
+				expression = b.arrow(
+					[b.id('$$node')],
+					b.call(
+						evaluated.is_function ? expression : b.logical('||', expression, b.id('$.noop')),
+						b.id('$$node')
+					)
+				);
 			}
 
 			push_prop(b.prop('get', b.call('$.attachment'), expression, true));

packages/svelte/src/compiler/phases/scope.js

@@ -20,6 +20,7 @@ const UNKNOWN = Symbol('unknown');
 /** Includes `BigInt` */
 export const NUMBER = Symbol('number');
 export const STRING = Symbol('string');
+export const FUNCTION = Symbol('string');
 
 /** @type {Record<string, [type: NUMBER | STRING | UNKNOWN, fn?: Function]>} */
 const globals = {
@@ -200,6 +201,13 @@ class Evaluation {
 	 */
 	is_number = true;
 
+	/**
+	 * True if the value is known to be a function
+	 * @readonly
+	 * @type {boolean}
+	 */
+	is_function = true;
+
 	/**
 	 * @readonly
 	 * @type {any}
@@ -209,7 +217,7 @@ class Evaluation {
 	/**
 	 *
 	 * @param {Scope} scope
-	 * @param {Expression} expression
+	 * @param {Expression | FunctionDeclaration} expression
 	 * @param {Set<any>} values
 	 */
 	constructor(scope, expression, values) {
@@ -500,6 +508,13 @@ class Evaluation {
 				break;
 			}
 
+			case 'ArrowFunctionExpression':
+			case 'FunctionExpression':
+			case 'FunctionDeclaration': {
+				this.values.add(FUNCTION);
+				break;
+			}
+
 			default: {
 				this.values.add(UNKNOWN);
 			}
@@ -516,6 +531,10 @@ class Evaluation {
 				this.is_number = false;
 			}
 
+			if (value !== FUNCTION) {
+				this.is_function = false;
+			}
+
 			if (value == null || value === UNKNOWN) {
 				this.is_defined = false;
 			}

packages/svelte/tests/runtime-runes/samples/attachment-component-falsy/Child.svelte

@@ -0,0 +1,5 @@
+<script>
+	let props = $props();
+</script>
+
+<div {...props}></div>

packages/svelte/tests/runtime-runes/samples/attachment-component-falsy/_config.js

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

packages/svelte/tests/runtime-runes/samples/attachment-component-falsy/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	import Child from './Child.svelte';
+
+	function attachment(){
+		console.log("up");
+	}
+
+	let enabled = $state(false);
+</script>
+
+<button onclick={() => enabled = !enabled}></button>
+
+<Child {@attach enabled && attachment} />