fix: allow multiple optional parameters with defaults

Author: paoloricciuti

.changeset/ten-geese-share.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: allow multiple optional parameters with defaults

packages/svelte/src/compiler/phases/1-parse/state/tag.js

@@ -281,7 +281,14 @@ function open(parser) {
 			parser.allow_whitespace();
 			if (parser.eat('=')) {
 				parser.allow_whitespace();
-				const right = read_expression(parser);
+				let right = read_expression(parser);
+				// multiple assignment expression will be confused by the parser for a sequence expression
+				// so if the returned value is a sequence expression we use the first expression as the
+				// right and we reset the parser.index
+				if (right.type === 'SequenceExpression') {
+					right = right.expressions[0];
+					parser.index = right.end ?? parser.index;
+				}
 				pattern = {
 					type: 'AssignmentPattern',
 					left: pattern,

packages/svelte/tests/runtime-runes/samples/snippet-optional-arguments-defaults/_config.js

@@ -0,0 +1,7 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: '012',
+	ssrHtml: '012'
+});

packages/svelte/tests/runtime-runes/samples/snippet-optional-arguments-defaults/main.svelte

@@ -0,0 +1,5 @@
+{#snippet snip(a, b = 1, c=2)}
+{a}{b}{c}
+{/snippet}
+
+{@render snip(0)}