From 8c750dadb8f063daf077187fc760322555762139 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Wed, 28 Jun 2023 16:50:08 +0200
Subject: [PATCH] fix: ensure createEventDispatcher works with types from
 generics

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`
---
 .changeset/long-humans-dress.md                       |  5 +++++
 documentation/docs/05-misc/03-typescript.md           |  2 +-
 documentation/docs/05-misc/04-v4-migration-guide.md   |  2 +-
 packages/svelte/src/runtime/internal/public.d.ts      | 10 ++++------
 packages/svelte/test/types/actions.ts                 |  2 +-
 packages/svelte/test/types/create-event-dispatcher.ts |  2 +-
 6 files changed, 13 insertions(+), 10 deletions(-)
 create mode 100644 .changeset/long-humans-dress.md

diff --git a/.changeset/long-humans-dress.md b/.changeset/long-humans-dress.md
new file mode 100644
index 000000000000..68502950f72e
--- /dev/null
+++ b/.changeset/long-humans-dress.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure createEventDispatcher works with types from generics
diff --git a/documentation/docs/05-misc/03-typescript.md b/documentation/docs/05-misc/03-typescript.md
index 36378365fd50..778e13ce37f4 100644
--- a/documentation/docs/05-misc/03-typescript.md
+++ b/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
 	}>();
diff --git a/documentation/docs/05-misc/04-v4-migration-guide.md b/documentation/docs/05-misc/04-v4-migration-guide.md
index 25faea67cead..81749a4ba969 100644
--- a/documentation/docs/05-misc/04-v4-migration-guide.md
+++ b/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:
diff --git a/packages/svelte/src/runtime/internal/public.d.ts b/packages/svelte/src/runtime/internal/public.d.ts
index 6eab766077b6..1f1011740d21 100644
--- a/packages/svelte/src/runtime/internal/public.d.ts
+++ b/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;
 }
diff --git a/packages/svelte/test/types/actions.ts b/packages/svelte/test/types/actions.ts
index bc1225a8ca20..03659e760540 100644
--- a/packages/svelte/test/types/actions.ts
+++ b/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
 
diff --git a/packages/svelte/test/types/create-event-dispatcher.ts b/packages/svelte/test/types/create-event-dispatcher.ts
index 31f06f71b56c..f85196930b58 100644
--- a/packages/svelte/test/types/create-event-dispatcher.ts
+++ b/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;