Make Reducer more polymorphic

Author: pelotom

index.d.ts

@@ -46,7 +46,7 @@ export interface Action<T = any> {
  * @template S The type of state consumed and produced by this reducer.
  * @template A The type of actions the reducer can potentially respond to.
  */
-export type Reducer<S = {}, A extends Action = Action> = (state: S, action: A) => S;
+export type Reducer<S = {}, A extends Action = Action> = <Act extends A>(state: S, action: Act) => S;
 
 /**
  * Object whose values correspond to different reducer functions.