fix: ensure createEventDispatcher works with types from generics

[dummdidumm]

fixes #8860

This contains a small but unfortunately unavoidable breaking change: If you used never to type that the second parameter shouldn't be set (which the docs recommended for a short time), then you need to change that to null:

const dispatch = createEventDispatcher<{
-  noParameter: never;
+  noParameter: null;
}>();

Same for ActionReturn - use undefined instead of never:

- function action(el: HTMLElement): ActionReturn<never> {
+ function action(el: HTMLElement): ActionReturn<undefined> {
  // ..
}

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[benmccann]

I will admit to having limited understanding of the explanation, but it sounds like this is necessary based on microsoft/TypeScript#54806, so lgtm

[dummdidumm]

We'll need to do the same for the Action interface 😞 will get to it later

[GauBen]

Hey! I observed a few interesting behaviors regarding the new createEventDispatcher type.

1. This pull request slightly changes the behavior for null and undefined.

   // Setup
   const dispatch = createEventDispatcher<{
   	optional?: number;
   	nullable: number | null;
   }>();

   Before

   // Both are ok
   dispatch('optional');
   dispatch('nullable');
   
   // Both are ok
   dispatch('optional', undefined);
   dispatch('nullable', undefined);
   
   // Inconsistency
   dispatch('optional', null); // Error
   dispatch('nullable', null); // Ok

   After

   // All ok
   dispatch('optional');
   dispatch('nullable');
   dispatch('optional', undefined);
   dispatch('nullable', undefined);
   dispatch('optional', null);
   dispatch('nullable', null);

2. As CustomEvent.detail defaults to null, the inferred types are slightly off when using undefined/?::

   const dispatch = createEventDispatcher<{ click: undefined }>();

   

   

   https://svelte.dev/repl/147bfc07869f4909b34f83951665155b?version=4.0.0

   Tracked in CustomEvent.detail is null when not defined microsoft/TypeScript-DOM-lib-generator#1585

I thought it was interesting to mention here, as tightening types might be a breaking change

[dummdidumm]

Thanks for testing these out! The new behavior is actually more correct, as undefined and null will both result in null for event.detail

The last insight is actually unrelated, it's how the Svelte language server resolves these types - that hasn't changed, but should probably adjusted to be null for detail in that case.