strictEvents event dispatchers can be called without providing a value when the event key is not optional

[DetachHead]

Describe the bug

if a key in an event dispatcher type is required, it can be dispatched without a value, causing the CustomEvent.detail to be null at runtime when typescript does not say it's nullable

Reproduction

a.svelte

<script lang="ts" strictEvents>
    import { createEventDispatcher } from 'svelte'

    const dispatch = createEventDispatcher<{ a: number }>()
</script>

<button on:click={() => dispatch('a')}>asdf</button>

b.svelte

<script lang="ts" strictEvents>
    import A from './a.svelte'
</script>

<A
    on:a={(e) => {
        // no typescript error, crashes at runtime
        console.log(e.detail.toString())
    }}
/>

Expected behaviour

the function created by createEventDispatcher should require the detail argument when the EventKey type is required.

the updated function signature could look something like this:

type RequiredKeys<T> = Exclude<{
    [K in keyof T]-?: undefined extends T[K] ? never : K;
}[keyof T], keyof {}>

type OptionalKeys<T> = Exclude<keyof T, RequiredKeys<T>>

declare function createEventDispatcher<EventMap extends {} = any>(): {
    <EventKey extends RequiredKeys<EventMap>>(type: EventKey, detail: EventMap[EventKey], options?: DispatchOptions): boolean;
    <EventKey extends OptionalKeys<EventMap>>(type: EventKey, detail?: EventMap[EventKey], options?: DispatchOptions): boolean;
}

playground

System Info

• OS: windows 10
• IDE: vscode

Which package is the issue about?

svelte-check

Additional Information, eg. Screenshots

No response

[jasonlyu123]

See upstream PR sveltejs/svelte#7224

[jasonlyu123]

Fixed in svelte 4