Using a generic type as an argument to a Snippet breaks types generation of a component

[tommyminds]

Describe the bug

When I use a generic type as an argument to a snippet, svelte-package does not properly generate a Component.d.ts file for my component anymore.

While I understand the following test case is a bit weird, I do have several components where I use the type of one of the properties to determine the type that is passed to subsequent snippets.

Reproduction

<script lang="ts" generics="T">
	import type { Snippet } from 'svelte';

	let {
		data,
		someSnippet
	}: {
		data: T;
		someSnippet?: Snippet<[T]>;
	} = $props();
</script>

<div>{@render someSnippet?.(data)}</div>

Logs

No response

System Info

System:
    OS: macOS 14.5
    CPU: (8) arm64 Apple M1 Pro
    Memory: 72.55 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.19.1 - ~/.nvm/versions/node/v18.19.1/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v18.19.1/bin/yarn
    npm: 10.2.4 - ~/.nvm/versions/node/v18.19.1/bin/npm
    pnpm: 9.3.0 - /opt/homebrew/bin/pnpm
    bun: 1.1.13 - /opt/homebrew/bin/bun
  Browsers:
    Chrome: 126.0.6478.127
    Safari: 17.5
  npmPackages:
    svelte: ^5.0.0-next.1 => 5.0.0-next.166

Severity

blocking an upgrade

[paoloricciuti]

I suspect this has something to do with language-tools I'll try to take a look later

[tommyminds]

I found other cases where types are not generated anymore. There also seems to be an issue using ComponentProps for components that have a Snippet on their props. For example the following causes the outer component to not generate types anymore.

Outer.svelte

<script lang="ts">
	import type { ComponentProps } from 'svelte';
	import Inner from './Inner.svelte';
	let { foo }: ComponentProps<Inner> = $props();
</script>

Inner.svelte

<script lang="ts">
	import { type Snippet } from 'svelte';
	let {
		foo,
		footer
	}: {
		foo: string;
		footer?: Snippet;
	} = $props();
</script>

[dummdidumm]

Investigating this. For some reason TypeScript seems to want to unpack the types when using the ComponentProps type, likely because of the infer used in there. That in combination with the generated d.ts output of the default export of components leads to TS breaking down somewhere. This not only happens for Snippet, it can happen for other imported types, too.
Not sure what a good fix could be yet.

[dummdidumm]

Investigated this further. Ultimately this is a TypeScript limitation: For reasons that I don't understand TypeScript is not able to correctly backtrack imports if the declaration file is written of the form

declare module "svelte" {
  class Foo {}
  export { Foo }
}

TypeScript will then error with something like Exported variable 'X' has or is using name 'Foo' from external module "svelte" but cannot be named.
If the module is declared like this instead

declare module "svelte" {
  export class Foo {}
  export {}
}

The error does not appear. It seems that the extra indirection is too much for TypeScript to know where to find the import and then errors.

The fix is to revert Rich-Harris/dts-buddy#82 in dts-buddy and instead add export {} at the end, which has the same outcome with regards to which types can be imported.