Error with component typedef doesn't have event

[jasonlyu123]

Describe the bug

There'll be a type check error when the Component type definition extends SvelteCopmonentTyped<{}, {}, {}> in strict mode. Not sure if we should count it as a bug because it's possible to avoid it in the userland.

The error is

Argument of type 'typeof Component' is not assignable to parameter of type 'ConstructorOfATypedSvelteComponent'.
  Types of construct signatures are incompatible.
    Type 'new (options: IComponentOptions<{}>) => Component' is not assignable to type 'new (args: { target: any; props?: any; }) => ATypedSvelteComponent'.
      Construct signature return types 'Component' and 'ATypedSvelteComponent' are incompatible.
        The types of '$on' are incompatible between these types.
          Type '<K extends never>(type: K, callback: (e: {}[K]) => void) => () => void' is not assignable to type '(event: string, handler: ((e: any) => any) | null | undefined) => () => void'.
            Types of parameters 'callback' and 'handler' are incompatible.
              Type '((e: any) => any) | null | undefined' is not assignable to type '(e: never) => void'.
                Type 'undefined' is not assignable to type '(e: never) => void'.
...

Reproduction

<script lang="ts">
    import { SvelteComponentTyped } from "svelte";

    class Component extends SvelteComponentTyped<{}, {}, {}>{}
</script>

<Component></Component>

Expected behaviour

no error.

System Info

• OS: Windows
• IDE: VSCode

Which package is the issue about?

Svelte for VS Code extension

Additional Information, eg. Screenshots

No response

[dummdidumm]

I can't reproduce only with earlier versions of Svelte that had the | undefined | null missing. I'm not sure what we can do about this except say "bump your Svelte version".

[jasonlyu123]

Yeah. Indeed it doesn't happen in the latest svelte version That was weird. I thought I am crazy when I can't reproduce it in the typescript playground. So we fixed it with some unrelated changes 😆. Still worth the effort for the visibility.

[jasonlyu123]

Closing since svelte-check now requires a later version that doesn't have this problem. If anyone encounter this you can update svelte to 3.51 or later.