fix(store): adjust types to allow a generic reducer (#2996)

Closes #2982

modules/store/spec/types/reducer_creator.spec.ts

@@ -4,7 +4,7 @@ import { compilerOptions } from './utils';
 describe('createReducer()', () => {
   const expectSnippet = expecter(
     (code) => `
-      import {createAction, createReducer, on, props} from '@ngrx/store';
+      import {createAction, createReducer, on, props, ActionCreator, ActionReducer} from '@ngrx/store';
 
       ${code}
     `,
@@ -57,6 +57,23 @@ describe('createReducer()', () => {
         );
       `).toInfer('reducer', 'ActionReducer<number, Action>');
     });
+
+    it('should support a generic reducer factory', () => {
+      expectSnippet(`
+        const creator = null as unknown as ActionCreator;
+
+        export function createGenericReducer<TState extends object>(initialState: TState): ActionReducer<TState> {
+          const reducer = createReducer(
+            initialState,
+            on(creator, (state, action) => {
+              return state ;
+            })
+          );
+
+          return reducer;
+        }
+      `).toSucceed();
+    });
   });
 
   describe('on()', () => {

modules/store/src/reducer_creator.ts

@@ -22,9 +22,7 @@ export interface ReducerTypes<
 
 // Specialized Reducer that is aware of the Action type it needs to handle
 export interface OnReducer<State, Creators extends readonly ActionCreator[]> {
-  (state: State, action: ActionType<Creators[number]>): State extends object
-    ? { [P in keyof State]: State[P] }
-    : State;
+  (state: State, action: ActionType<Creators[number]>): State;
 }
 
 /**
@@ -42,11 +40,14 @@ export interface OnReducer<State, Creators extends readonly ActionCreator[]> {
  * ```
  */
 export function on<State, Creators extends readonly ActionCreator[]>(
-  ...args: [...creators: Creators, reducer: OnReducer<State, Creators>]
+  ...args: [
+    ...creators: Creators,
+    reducer: OnReducer<State extends infer S ? S : never, Creators>
+  ]
 ): ReducerTypes<State, Creators> {
   // This could be refactored when TS releases the version with this fix:
   // https://github.com/microsoft/TypeScript/pull/41544
-  const reducer = args.pop() as OnReducer<State, Creators>;
+  const reducer = args.pop() as OnReducer<any, Creators>;
   const types = (((args as unknown) as Creators).map(
     (creator) => creator.type
   ) as unknown) as ExtractActionTypes<Creators>;