type T1<T> = T extends () => infer p ? p : number 

type T2 = T1<() => string>
//T2 === string
type T3 = T1<(i: number) => void>
//T3 === number

type T4<T> = T extends (a: infer P, b: infer P) => void ? P : any;
type T5 = T4<() => void>; //any
type T6 = T4<(a:string, b: number) => void> // string & number

type User = { name: string, age:number };
type OtherInfo = { sex: string };
type T7 = T4<(a: User, b:OtherInfo) => void> // User & OtherInfo

type ReturnType<T extends (...args: any) => any> = T 
  extends (...args: any) => infer R ? R : any;

/**
 * Infer a combined state shape from a `ReducersMapObject`.
 *
 * @template M Object map of reducers as provided to `combineReducers(map: M)`.
 */
export type StateFromReducersMapObject<M> = M extends ReducersMapObject
  ? { [P in keyof M]: M[P] extends Reducer<infer S, any> ? S : never }
  : never
  
 

/**
 * Infer reducer union type from a `ReducersMapObject`.
 *
 * @template M Object map of reducers as provided to `combineReducers(map: M)`.
 */
export type ReducerFromReducersMapObject<M> = M extends {
  [P in keyof M]: infer R
}
  ? R extends Reducer<any, any>
    ? R
    : never
  : never

/**
 * Infer action type from a reducer function.
 *
 * @template R Type of reducer.
 */
export type ActionFromReducer<R> = R extends Reducer<any, infer A> ? A : never