[fix] #6823 - some properties in type of object returned from ActionD…

…ata is missing (#6869)

Fixes #6823
Fixes #6822

Co-authored-by: William Viktorsson <wilvik05@edu.umea.se>
Co-authored-by: williamv <williamv@cs.umu.se>

.changeset/breezy-pots-wave.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix to ActionData type generation

packages/kit/types/index.d.ts

@@ -35,9 +35,20 @@ export type AwaitedProperties<input extends Record<string, any> | void> = input
 	? input
 	: unknown;
 
-export type AwaitedActions<T extends Record<string, (...args: any) => any>> = {
-	[Key in keyof T]: UnpackValidationError<Awaited<ReturnType<T[Key]>>>;
-}[keyof T];
+export type AwaitedActions<T extends Record<string, (...args: any) => any>> = Expand<
+	{
+		[Key in keyof T]: OptionalUnion<UnpackValidationError<Awaited<ReturnType<T[Key]>>>>;
+	}[keyof T]
+>;
+
+// Makes sure a type is "repackaged" and therefore more readable
+type Expand<T> = T extends infer O ? { [K in keyof O]: O[K] } : never;
+// Takes a union type and returns a union type where each type also has all properties
+// of all possible types (typed as undefined), making accessing them more ergonomic
+type OptionalUnion<
+	U extends Record<string, any>, // not unknown, else interfaces don't satisfy this constraint
+	A extends keyof U = U extends U ? keyof U : never
+> = U extends unknown ? { [P in Exclude<A, keyof U>]?: never } & U : never;
 
 // Needs to be here, else ActionData will be resolved to unknown - probably because of "d.ts file imports .js file" in combination with allowJs
 interface ValidationError<T extends Record<string, unknown> | undefined = undefined> {