Svelte 5: Types are always required for snippet props if Typescript is used in any component.

[gulbanana]

Describe the bug

I've created a Svelte 5 project and enabled typescript. However, I'm not using lang="ts" in every <script> block. Some components' types are unimportant or can be fully inferred. Unfortunately, this one doesn't compile:

<script>
    let {whom} = $props();
</script>

<p>Hello, {@render whom()}.</p>

Here's a screenshot of the error.

If I fully type everything, it works:

<script lang="ts">
    import type { Snippet } from 'svelte';
    let {whom}: {whom: Snippet} = $props();
</script>

<p>Hello, {@render whom()}.</p>

That's a lot of extra boilerplate, though. You've got the type import and then the stuttering destructure. Is it a bug that lang="ts" is required in this case?

Reproduction

1. Clone https://github.com/gulbanana-bugs/repro-svelte-required-typing
2. Open in Visual Studio Code or any other editor with Typescript integration.
3. Observe that Greet2.svelte has an error, but Greet.svelte does not.

Logs

No response

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (20) x64 12th Gen Intel(R) Core(TM) i7-12700
    Memory: 15.64 GB / 31.77 GB
  Binaries:
    Node: 20.11.1 - C:\Program Files\nodejs\node.EXE
    npm: 10.2.4 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Chromium (127.0.2651.74)
    Internet Explorer: 11.0.19041.4355
  npmPackages:
    svelte: ^5.0.0 => 5.0.5

Severity

annoyance

[david-plugge]

Sounds like checkJS is true inside the tsconfig

[gulbanana]

Yes, it is. If that's turned off, will it disable typechecking in the html part of svelte files that do use lang="ts"?

[david-plugge]

Im not sure, just try and see what happens.
Id assume lang=ts enables typescript for the entire file

[dummdidumm]

This is not a bug, it's how you have to type it. Yes it's more to write in this case but it will be easier to see at a glance what the component's public API is when you read (no jumping between where props are declared and where they are used). Therefore closing.