fix: correctly validate undefined snippet params with default value

.changeset/angry-mayflies-matter.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly validate `undefined` snippet params with default value

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

@@ -21,6 +21,10 @@ export function SnippetBlock(node, context) {
 	/** @type {Statement[]} */
 	const declarations = [];
 
+	if (dev) {
+		declarations.push(b.stmt(b.call('$.validate_snippet_args', b.spread(b.id('arguments')))));
+	}
+
 	const transform = { ...context.state.transform };
 	const child_state = { ...context.state, transform };
 
@@ -30,12 +34,7 @@ export function SnippetBlock(node, context) {
 		if (!argument) continue;
 
 		if (argument.type === 'Identifier') {
-			args.push({
-				type: 'AssignmentPattern',
-				left: argument,
-				right: b.id('$.noop')
-			});
-
+			args.push(b.assignment_pattern(argument, b.id('$.noop')));
 			transform[argument.name] = { read: b.call };
 
 			continue;
@@ -66,29 +65,16 @@ export function SnippetBlock(node, context) {
 			}
 		}
 	}
-	if (dev) {
-		declarations.unshift(
-			b.stmt(
-				b.call(
-					'$.validate_snippet_args',
-					.../** @type {Identifier[]} */ (
-						args.map((arg) => (arg?.type === 'Identifier' ? arg : arg?.left))
-					)
-				)
-			)
-		);
-	}
+
 	body = b.block([
 		...declarations,
 		.../** @type {BlockStatement} */ (context.visit(node.body, child_state)).body
 	]);
 
-	/** @type {Expression} */
-	let snippet = b.arrow(args, body);
-
-	if (dev) {
-		snippet = b.call('$.wrap_snippet', b.id(context.state.analysis.name), snippet);
-	}
+	// in dev we use a FunctionExpression (not arrow function) so we can use `arguments`
+	let snippet = dev
+		? b.call('$.wrap_snippet', b.id(context.state.analysis.name), b.function(null, args, body))
+		: b.arrow(args, body);
 
 	const declaration = b.const(node.expression, snippet);
 

packages/svelte/tests/runtime-runes/samples/validate-undefined-snippet-default-arg/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `<p>default</p>`
+});

packages/svelte/tests/runtime-runes/samples/validate-undefined-snippet-default-arg/main.svelte

@@ -0,0 +1,5 @@
+{#snippet test(param = "default")}
+    <p>{param}</p>
+{/snippet}
+
+{@render test()}