Snippets and their argument types not correctly resolved from generic legacy components

[brunnerh]

Describe the bug

Using a legacy component with slots via snippets, the available snippets may not be resolved correctly and arguments passed to the snippets end up as any.

Reproduction

<!-- loader.svelte -->
<script lang="ts" generics="T">
	interface $$Slots {
		default: { value: T },
		loaded: { value: T },
		loading: { },
		error: { error: any },
	}

	export let promise: Promise<T>;
</script>

{#await promise}
	<slot name="loading">
		Loading...
	</slot>
{:then value}
	<slot {value} />
	<slot name="loaded" {value} />
{:catch}
  Error!
{/await}

<script lang="ts">
  import Loader from './loader.svelte';

  const delay = (ms: number) => new Promise(resolve => setTimeout(() => resolve(ms), ms));

  const promise = delay(1000).then(() => "resolved");
</script>

<Loader promise={promise}>
  <!--
    Error on `loading`:
    Object literal may only specify known properties, and 'loading' does not exist
    in type '{ promise: Promise<string>; } & { children?: any; }'. ts(2353)
  -->
  {#snippet loading()}
    Loading text...
  {/snippet}

  <!--
    Error on `value`:
    <!-- Binding element 'value' implicitly has an 'any' type. ts(7031)
  -->
  {#snippet loaded({ value })}
    Text: {value}
  {/snippet}

  {#snippet error()}
    Error.
  {/snippet}
</Loader>

(The runtime behavior is correct.)

Expected behaviour

• loading is detected as a valid slot/snippet.
• value is generically typed correctly (here string).

System Info

• OS: Windows 10, 64bit
• IDE: VSCode 1.98.2

Which package is the issue about?

Svelte for VS Code extension

Additional Information, eg. Screenshots

Extension & svelte-check yield same errors.

If the loading snippet is not there, the next snippet will show an error, only promise & children are detected as valid properties.