fix: handle multiple snippet parameters with one or more being optional (#10833)

fixes #10825

Co-authored-by: Dominic Gannaway <dg@domgan.com>

.changeset/slow-plums-chew.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: handle multiple snippet parameters with one or more being optional

packages/svelte/src/compiler/phases/1-parse/read/context.js

@@ -121,7 +121,10 @@ function read_type_annotation(parser, optional_allowed = false) {
 	const template =
 		parser.template.slice(0, a).replace(/[^\n]/g, ' ') +
 		insert +
-		parser.template.slice(parser.index);
+		// If this is a type annotation for a function parameter, Acorn-TS will treat subsequent
+		// parameters as part of a sequence expression instead, and will then error on optional
+		// parameters (`?:`). Therefore replace that sequence with something that will not error.
+		parser.template.slice(parser.index).replace(/\?\s*:/g, ':');
 	let expression = parse_expression_at(template, parser.ts, a);
 
 	// `foo: bar = baz` gets mangled — fix it

packages/svelte/tests/runtime-runes/samples/snippet-typescript/_config.js

@@ -1,5 +1,5 @@
 import { test } from '../../test';
 
 export default test({
-	html: '1 2 3 4 5'
+	html: '1 2 3 4 5 6a'
 });

packages/svelte/tests/runtime-runes/samples/snippet-typescript/main.svelte

@@ -16,9 +16,13 @@
 {#snippet counter5(c?: number = 5)}
 	{c}
 {/snippet}
+{#snippet counter6(c?: number, d?: string)}
+	{c}{d}
+{/snippet}
 
 {@render counter1(1)}
 {@render counter2({ c: 2 })}
 {@render counter3(3)}
 {@render counter4()}
 {@render counter5()}
+{@render counter6(6, 'a')}