Compiler fails parsing template literal types in TS script blocks (js_parse_error)

[pheuter]

Summary

Svelte compiler fails to parse TypeScript template literal types in <script lang="ts">, producing js_parse_error Unexpected token. This happens even without runes or $derived.

Repro

Create any Svelte/SvelteKit project (I used sv create + bun install) and put this in a .svelte file:

<script lang="ts">
  let x = ['csv', 'json'] as (`csv` | `json`)[ ];
</script>

Then compile with the Svelte compiler directly:

import { compile } from 'svelte/compiler';
import { readFileSync } from 'node:fs';

const source = readFileSync('./src/routes/+page.svelte', 'utf8');

try {
  const result = compile(source, {
    filename: 'src/routes/+page.svelte',
    generate: 'client',
    dev: true,
    runes: true
  });
  console.log('ok', result.warnings?.length ?? 0);
} catch (e) {
  console.error(e?.code, e?.message);
  if (e?.start) console.error(JSON.stringify(e.start));
  process.exit(1);
}

Actual

js_parse_error Unexpected token with a location pointing at the template literal type.

Expected

Template literal types are valid TS and should parse.

Notes

This also fails when embedded in $derived, but $derived is not required to reproduce.

Environment

• Svelte: 5.48.2 (from sv create)
• bun: 1.3.6
• OS: macOS (arm64)

[pheuter]

Originated from svelte-check-rs issue pheuter/svelte-check-rs#84.

[paoloricciuti]

The issue is not with template strings but with (\`` if you remove the parentheses it works. I'll move this to typescript-acorn` repo as it's an issue with the Typescript parser.