fix: ensure createEventDispatcher and ActionReturn work with generic function types (#8872)

fixes #8860

This contains a small but unfortunately unavoidable breaking change: If you used `never` to type that the second parameter of `createEventDispatcher` shouldn't be set or that the action accepts no parameters (which the docs recommended for a short time), then you need to change that to `null` and `undefined` respectively

Author: dummdidumm

.changeset/long-humans-dress.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure `createEventDispatcher` and `ActionReturn` work with types from generic function parameters

documentation/docs/05-misc/03-typescript.md

@@ -96,7 +96,7 @@ Events can be typed with `createEventDispatcher`:
 	import { createEventDispatcher } from 'svelte';
 
 	const dispatch = createEventDispatcher<{
-		event: never; // does not accept a payload
+		event: null; // does not accept a payload
 		type: string; // has a required string payload
 		click: string | null; // has an optional string payload
 	}>();

documentation/docs/05-misc/04-v4-migration-guide.md

@@ -36,7 +36,7 @@ import { createEventDispatcher } from 'svelte';
 const dispatch = createEventDispatcher<{
 	optional: number | null;
 	required: string;
-	noArgument: never;
+	noArgument: null;
 }>();
 
 // Svelte version 3:
@@ -50,10 +50,10 @@ dispatch('required'); // error, missing argument
 dispatch('noArgument', 'surprise'); // error, cannot pass an argument
 ```
 
-- `Action` and `ActionReturn` have a default parameter type of `never` now, which means you need to type the generic if you want to specify that this action receives a parameter. The migration script will migrate this automatically ([#7442](https://github.com/sveltejs/svelte/pull/7442))
+- `Action` and `ActionReturn` have a default parameter type of `undefined` now, which means you need to type the generic if you want to specify that this action receives a parameter. The migration script will migrate this automatically ([#7442](https://github.com/sveltejs/svelte/pull/7442))
 
 ```diff
--const action: Action = (node, params) => { .. } // this is now an error, as params is expected to not exist
+-const action: Action = (node, params) => { .. } // this is now an error if you use params in any way
 +const action: Action<HTMLElement, string> = (node, params) => { .. } // params is of type string
 ```
 

packages/svelte/src/runtime/action/public.d.ts

@@ -1,8 +1,8 @@
 /**
  * Actions can return an object containing the two properties defined in this interface. Both are optional.
  * - update: An action can have a parameter. This method will be called whenever that parameter changes,
- *   immediately after Svelte has applied updates to the markup. `ActionReturn` and `ActionReturn<never>` both
- *   mean that the action accepts no parameters, which makes it illegal to set the `update` method.
+ *   immediately after Svelte has applied updates to the markup. `ActionReturn` and `ActionReturn<undefined>` both
+ *   mean that the action accepts no parameters.
  * - destroy: Method that is called after the element is unmounted
  *
  * Additionally, you can specify which additional attributes and events the action enables on the applied element.
@@ -27,10 +27,10 @@
  * Docs: https://svelte.dev/docs/svelte-action
  */
 export interface ActionReturn<
-	Parameter = never,
+	Parameter = undefined,
 	Attributes extends Record<string, any> = Record<never, any>
 > {
-	update?: [Parameter] extends [never] ? never : (parameter: Parameter) => void;
+	update?: (parameter: Parameter) => void;
 	destroy?: () => void;
 	/**
 	 * ### DO NOT USE THIS
@@ -50,7 +50,7 @@ export interface ActionReturn<
  *   // ...
  * }
  * ```
- * `Action<HTMLDivElement>` and `Action<HTMLDiveElement, never>` both signal that the action accepts no parameters.
+ * `Action<HTMLDivElement>` and `Action<HTMLDiveElement, undefined>` both signal that the action accepts no parameters.
  *
  * You can return an object with methods `update` and `destroy` from the function and type which additional attributes and events it has.
  * See interface `ActionReturn` for more details.
@@ -59,18 +59,15 @@ export interface ActionReturn<
  */
 export interface Action<
 	Element = HTMLElement,
-	Parameter = never,
+	Parameter = undefined,
 	Attributes extends Record<string, any> = Record<never, any>
 > {
 	<Node extends Element>(
-		...args: [Parameter] extends [never]
-			? [node: Node]
-			: undefined extends Parameter
+		...args: undefined extends Parameter
 			? [node: Node, parameter?: Parameter]
 			: [node: Node, parameter: Parameter]
 	): void | ActionReturn<Parameter, Attributes>;
 }
 
 // Implementation notes:
 // - undefined extends X instead of X extends undefined makes this work better with both strict and nonstrict mode
-// - [X] extends [never] is needed, X extends never would reduce the whole resulting type to never and not to one of the condition outcomes

packages/svelte/src/runtime/internal/public.d.ts

@@ -80,14 +80,12 @@ export interface DispatchOptions {
 export interface EventDispatcher<EventMap extends Record<string, any>> {
 	// Implementation notes:
 	// - undefined extends X instead of X extends undefined makes this work better with both strict and nonstrict mode
-	// - [X] extends [never] is needed, X extends never would reduce the whole resulting type to never and not to one of the condition outcomes
+	// - | null | undefined is added for convenience, as they are equivalent for the custom event constructor (both result in a null detail)
 	<Type extends keyof EventMap>(
-		...args: [EventMap[Type]] extends [never]
-			? [type: Type, parameter?: null | undefined, options?: DispatchOptions]
-			: null extends EventMap[Type]
-			? [type: Type, parameter?: EventMap[Type], options?: DispatchOptions]
+		...args: null extends EventMap[Type]
+			? [type: Type, parameter?: EventMap[Type] | null | undefined, options?: DispatchOptions]
 			: undefined extends EventMap[Type]
-			? [type: Type, parameter?: EventMap[Type], options?: DispatchOptions]
+			? [type: Type, parameter?: EventMap[Type] | null | undefined, options?: DispatchOptions]
 			: [type: Type, parameter: EventMap[Type], options?: DispatchOptions]
 	): boolean;
 }

packages/svelte/test/types/actions.ts

@@ -1,4 +1,4 @@
-import type { Action, ActionReturn } from '$runtime/action';
+import type { Action, ActionReturn } from '$runtime/action/public';
 
 // ---------------- Action
 
@@ -65,30 +65,27 @@ const optional4: Action<HTMLElement, boolean | undefined> = (_node, _param?) =>
 };
 optional4;
 
-const no: Action<HTMLElement, never> = (_node) => {};
+const no: Action<HTMLElement, undefined> = (_node) => {};
 // @ts-expect-error second param
 no(null as any, true);
 no(null as any);
 // @ts-expect-error second param
 no(null as any, 'string');
 
-const no1: Action<HTMLElement, never> = (_node) => {
+const no1: Action<HTMLElement, undefined> = (_node) => {
 	return {
 		destroy: () => {}
 	};
 };
 no1;
 
-// @ts-expect-error param given
-const no2: Action<HTMLElement, never> = (_node, _param?) => {};
-no2;
+const no2: Action<HTMLElement, undefined> = (_node, _param?) => {};
+no2(null as any);
 
-// @ts-expect-error param given
-const no3: Action<HTMLElement, never> = (_node, _param) => {};
+const no3: Action<HTMLElement, undefined> = (_node, _param) => {};
 no3;
 
-// @ts-expect-error update method given
-const no4: Action<HTMLElement, never> = (_node) => {
+const no4: Action<HTMLElement, undefined> = (_node) => {
 	return {
 		update: () => {},
 		destroy: () => {}
@@ -106,7 +103,7 @@ requiredReturn;
 const optionalReturn: ActionReturn<boolean | undefined> = {
 	update: (p) => {
 		p === true;
-		// @ts-expect-error could be undefined
+		// @ts-expect-error (only in strict mode) could be undefined
 		p.toString();
 	}
 };
@@ -118,7 +115,7 @@ const invalidProperty: ActionReturn = {
 };
 invalidProperty;
 
-type Attributes = ActionReturn<never, { a: string }>['$$_attributes'];
+type Attributes = ActionReturn<undefined, { a: string }>['$$_attributes'];
 const attributes: Attributes = { a: 'a' };
 attributes;
 // @ts-expect-error wrong type

packages/svelte/test/types/create-event-dispatcher.ts

@@ -1,7 +1,7 @@
 import { createEventDispatcher } from '$runtime/internal/lifecycle';
 
 const dispatch = createEventDispatcher<{
-	loaded: never;
+	loaded: null;
 	change: string;
 	valid: boolean;
 	optional: number | null;