Typescript each indexed (#9567)

* failing test

* fix undefined index bug

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

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

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

@@ -106,7 +106,12 @@ function read_type_annotation(parser) {
 		const insert = '_ as ';
 		let a = parser.index - insert.length;
 		const template = ' '.repeat(a) + insert + parser.template.slice(parser.index);
-		const expression = parse_expression_at(template, parser.ts, a);
+		let expression = parse_expression_at(template, parser.ts, a);
+
+		// `array as item: string, index` becomes `string, index`, which is mistaken as a sequence expression - fix that
+		if (expression.type === 'SequenceExpression') {
+			expression = expression.expressions[0];
+		}
 
 		parser.index = /** @type {number} */ (expression.end);
 		return /** @type {any} */ (expression).typeAnnotation;

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

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	html: `
+		<span>0: a</span>
+		<span>1: b</span>
+		<span>2: c</span>
+	`
+});

packages/svelte/tests/runtime-runes/samples/typescript-each-indexed/main.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+	let letters = ['a', 'b', 'c'];
+</script>
+
+{#each letters as letter: string, i}
+	<span>{i}: {letter}</span>
+{/each}