fix: preserve correct parent pointers when stripping types from AST

fixes #14204

.changeset/lovely-mirrors-exercise.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: preserve correct parent pointers when stripping types from AST

packages/svelte/src/compiler/phases/1-parse/remove_typescript_nodes.js

@@ -131,10 +131,12 @@ const visitors = {
 };
 
 /**
+ * Removes all Typescript nodes from the AST that were added by the Acorn-TS-Plugin.
+ * Note that this MUTATES the AST, in order to correctly preserve the parent pointers.
  * @template T
  * @param {T} ast
  * @returns {T}
  */
 export function remove_typescript_nodes(ast) {
-	return walk(ast, null, visitors);
+	return walk(ast, null, visitors, { mutate: true });
 }

packages/svelte/tests/parser-modern/test.ts

@@ -1,8 +1,10 @@
 import * as fs from 'node:fs';
 import { assert, it } from 'vitest';
 import { parse } from 'svelte/compiler';
+import { parse as internal_parse } from '../../src/compiler/phases/1-parse/index.js';
 import { try_load_json } from '../helpers.js';
 import { suite, type BaseTest } from '../suite.js';
+import { remove_typescript_nodes } from '../../src/compiler/phases/1-parse/remove_typescript_nodes.js';
 
 interface ParserTest extends BaseTest {}
 
@@ -55,3 +57,16 @@ it('Strips BOM from the input', () => {
 		]
 	});
 });
+
+it('Preserves correct parent pointers after stripping TypeScript nodes', () => {
+	const input = `<script lang="ts"></script>
+	
+	<div>
+		<p class={x as y}>hi</p>
+	</div>
+	`;
+	const prev: any = internal_parse(input);
+	const post: any = remove_typescript_nodes(prev);
+	assert.equal(prev, post);
+	assert.equal(post.fragment.nodes[1].fragment.nodes[1].parent, post.fragment.nodes[1]);
+});