fix: don't strip lang="ts" tag in Svelte 5 (#12080)

* fix: don't strip `lang="ts"` tag in Svelte 5

closes sveltejs/svelte#10766

* rename parameters

---------

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

.changeset/loud-pens-mate.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/package": patch
+---
+
+fix: don't strip `lang="ts"` tag in Svelte 5

packages/package/src/utils.js

@@ -1,7 +1,10 @@
 import * as fs from 'node:fs';
 import * as path from 'node:path';
+import { VERSION } from 'svelte/compiler';
 import { posixify, mkdirp, walk } from './filesystem.js';
 
+const is_svelte_5_plus = Number(VERSION.split('.')[0]) >= 5;
+
 /**
  * Resolves aliases
  *
@@ -45,8 +48,13 @@ export function strip_lang_tags(content) {
 	return content
 		.replace(
 			/(<!--[^]*?-->)|(<script[^>]*?)\s(?:type|lang)=(["'])(.*?)\3/g,
-			// things like application/ld+json should be kept as-is. Preprocessed languages are "ts" etc
-			(match, s1, s2, _, s4) => (s4?.startsWith('application/') ? match : (s1 ?? '') + (s2 ?? ''))
+			// Things like application/ld+json should be kept as-is. Preprocessed languages are "ts" etc.
+			// Svelte 5 deals with TypeScript natively, and in the template, too, therefore keep it in.
+			// Not removing it would mean Svelte parses without its TS plugin and then runs into errors.
+			(match, comment, tag_open, _, type) =>
+				type?.startsWith('application/') || (is_svelte_5_plus && type === 'ts')
+					? match
+					: (comment ?? '') + (tag_open ?? '')
 		)
 		.replace(/(<!--[^]*?-->)|(<style[^>]*?)\s(?:type|lang)=(["']).*?\3/g, '$1$2');
 }