Snippet exports cause errors in VSCode

[HanielU]

Describe the bug

In VSCode the svelte language server breaks when exporting a snippet that:

• makes use of a Svelte component that spreads the props gotten as an argument
• uses an #if statement referencing an "unknown" identifier like Error (i.e it can't find it in the file scope)

Reproduction

<!-- App.svelte -->
<script lang="ts" module>
  import Dummy from "./DummyComponent.svelte";
  import type { HTMLAttributes } from "svelte/elements";
  
  export { errorBoundary }; // ERROR HERE: cannot find name 'errorBoundary'
  export { WithProps }; // ERROR HERE: cannot find name 'WithProps'
</script>

{#snippet errorBoundary(error: unknown, reset: () => void)}
  <Dummy class="text-red">This is a boundary</Dummy>
  
  {#if error instanceof Error}
    <p>This is causing the error in the module script</p>
  {/if}
  
  <button onclick={reset}>Reset</button>
{/snippet}

{#snippet WithProps(props: HTMLAttributes<HTMLDivElement>)}
  <Dummy {...props}>
    <h1>This is a snippet with Props</h1>
  </Dummy>
{/snippet}

<!-- DummyComponent.svelte -->
<script lang="ts">
  let {children, ...rest} = $props()
</script>

<div {...rest}>
  {@render children()}
</div>

Expected behaviour

There shouldn't be an error because the compiler doesn't break as shown in this REPL

System Info

• OS: Windows 11 Pro
• IDE: VSCode v1.93.1

Which package is the issue about?

Svelte for VS Code extension, svelte-language-server

Additional Information, eg. Screenshots

[Wombosvideo]

Additions to the bug description

More generally it seems to happen when trying to export snippets that reference any declaration defined outside¹ of the snippet.
This makes sense for declarations defined in the component's instance². This should not happen for snippets that do not reference such declarations but reference declarations from the <script module>.

I've ran into this issue when trying to create formatter snippets, with one formatter converting from svx³ resulting in the mentioned Cannot find name 'withDependencies' TypeScript error. The error does not impact compilation and everything works as expected.

Reproduction

<script lang="ts" module>
  import { compile } from 'mdsvex';

  // Fix for mdsvex: (https://stackoverflow.com/a/73396215)
  if (typeof globalThis.window !== 'undefined') {
    window.process = {
      cwd: () => '/'
    } as NodeJS.Process;
  }

  export { noDependencies }; // Works just fine
  export { withDependencies }; // ts(2304): Cannot find name 'withDependencies'.
</script>

{#snippet noDependencies(content: string)}
  {content}
{/snippet}

{#snippet withDependencies(content: string)}
  {#await compile(content) then compiled}
    {@html compiled?.code}
  {/await}
{/snippet}

Expected behaviour

• The error only occurs if a non-module¹ declaration is used.
• The error is more descriptive. Example: Cannot export snippets referencing variables defined outside the <script module>

Footnotes

1. a declaration that is not defined in the <script module>, i.e. a declaration that is neither a snippet parameter nor a @const declaration within the snippet's body. ↩ ↩²

2. i.e. imports or variables in the <script> tag, or other top-level snippets depending on non-module declarations ↩

3. a file format combining Markdown and Svelte. See https://github.com/pngwn/mdsvex ↩