svelte-check errors when handling custom event with periods in its name

[theodorejb]

Describe the bug

svelte-check outputs parse errors and type errors when a component handles custom events with a period in their name:

c:\code\main\src\routes\home\+page.svelte:189:8
Error: Identifier expected. (ts)

c:\code\main\src\routes\home\+page.svelte:189:86
Error: Unexpected token. Did you mean `{'>'}` or `>`? (ts)

c:\code\main\src\routes\home\+page.svelte:189:89
Error: Expected corresponding closing tag for JSX fragment. (ts)

c:\code\main\src\routes\home\+page.svelte:189:6
Error: Type '{ onhidden: true; }' is not assignable to type 'HTMLProps<HTMLDivElement>'.
  Property 'onhidden' does not exist on type 'HTMLProps<HTMLDivElement>'. Did you mean 'hidden'? (ts)

Reproduction

I added the following to my app.d.ts file per the comments in #1563:

declare namespace svelte.JSX {
    interface HTMLAttributes<T> {
        "onhidden.bs.modal"?: (e: CustomEvent) => void;
    }
}

In my component I have this Bootstrap modal element:

<div on:hidden.bs.modal={handleHide} class="modal" id="videoModal" aria-hidden="true"></div>

VSCode doesn't show any errors for this code, and when I build my app the event is handled as expected. However, when I run svelte-check, it produces 4 parse/type errors from the above line in the component.

Expected behaviour

svelte-check should be able to parse custom event names containing periods, and not output errors.

System Info

• OS: Windows 11
• IDE: VSCode

Which package is the issue about?

svelte-check

Additional Information, eg. Screenshots

No response

[jasonlyu123]

Did you use the --use-new-transformation option as mentioned in #1563 (comment)?

[theodorejb]

@jasonlyu123 No, I somehow missed that the --use-new-transformation flag had to be added to the svelte-check script. That does indeed make the error go away! Is there a reason it isn't enabled by default?

[jasonlyu123]

It'll be soon. You can see the new transformation roadmap here #1552. We want to test out the new typing format before adding it to the svelte core. And there'll be a new major version for svelte-check which enable new transformation by default.