Use undefined instead of never

Author: JoshuaKGoldberg

index.d.ts

@@ -1,5 +1,5 @@
 export declare namespace Base {
-  interface Options {}
+  interface Options { }
 }
 
 declare type ApiExtension = {
@@ -23,33 +23,34 @@ declare type UnionToIntersection<Union> = (
 declare type AnyFunction = (...args: any) => any;
 declare type ReturnTypeOf<T extends AnyFunction | AnyFunction[]> =
   T extends AnyFunction
-    ? ReturnType<T>
-    : T extends AnyFunction[]
-    ? UnionToIntersection<Exclude<ReturnType<T[number]>, void>>
-    : never;
+  ? ReturnType<T>
+  : T extends AnyFunction[]
+  ? UnionToIntersection<Exclude<ReturnType<T[number]>, void>>
+  : never;
 
 type ClassWithPlugins = Constructor<any> & {
   plugins: Plugin[];
 };
 
 type RemainingRequirements<PredefinedOptions> =
   keyof PredefinedOptions extends never
-    ? Base.Options
-    : Omit<Base.Options, keyof PredefinedOptions>
+  ? Base.Options
+  : Omit<Base.Options, keyof PredefinedOptions>
 
 type NonOptionalKeys<Obj> = {
-  [K in keyof Obj]: {} extends Pick<Obj, K> ? never : K;
+  [K in keyof Obj]: {} extends Pick<Obj, K> ? undefined : K;
 }[keyof Obj];
 
-type RequiredIfRemaining<PredefinedOptions, NowProvided> = 
-  NonOptionalKeys<RemainingRequirements<PredefinedOptions>> extends never
-    ? [(Partial<Base.Options> & NowProvided)?]
-    : [Partial<Base.Options> & RemainingRequirements<PredefinedOptions> & NowProvided];
+type RequiredIfRemaining<PredefinedOptions, NowProvided> =
+  NonOptionalKeys<RemainingRequirements<PredefinedOptions>> extends undefined
+  ? [(Partial<Base.Options> & NowProvided)?]
+  : [Partial<Base.Options> & RemainingRequirements<PredefinedOptions> & NowProvided];
 
 type ConstructorRequiringVersion<Class extends ClassWithPlugins, PredefinedOptions> = {
   defaultOptions: PredefinedOptions;
 } & {
   new <NowProvided>(...options: RequiredIfRemaining<PredefinedOptions, NowProvided>): Class & {
+    debugKeys: NonOptionalKeys<RemainingRequirements<PredefinedOptions>>;
     options: NowProvided & PredefinedOptions;
   };
 };
@@ -79,9 +80,9 @@ export declare class Base<TOptions extends Base.Options = Base.Options> {
   static plugin<
     Class extends ClassWithPlugins,
     Plugins extends [Plugin, ...Plugin[]],
-  >(
-    this: Class,
-    ...plugins: Plugins,
+    >(
+      this: Class,
+      ...plugins: Plugins,
   ): Class & {
     plugins: [...Class['plugins'], ...Plugins];
   } & Constructor<UnionToIntersection<ReturnTypeOf<Plugins>>>;
@@ -135,4 +136,4 @@ export declare class Base<TOptions extends Base.Options = Base.Options> {
 
   constructor(options: TOptions);
 }
-export {};
+export { };