//i'd like access to the inner generic arguments, without having to pass TPayload
type Unpromise<TPayload, TPromise extend Promise<TPayload>> = TPayload;

//and then use that also as a type guard for mapped types
type Reducer<TActions> = {
    [K in keyof TActions where TActions[K] extends Promise<any>]: (payload: Unpromise<TActions[K]>)=>void;
    [K in keyof TActions where TActions[K] !extends Promise<any>]: (payload: TActions[K] )=>void;
}

//redux style actions class
class Actions{
    get(): Promise<string>;
}
//redux style reducer
class MyReducer{
  get(payload:string){...}
}

//unpromise type
type Unpromise<TPayload, TPromise extend Promise<TPayload>> = TPayload;

//example derived usage with generic constraints
type Reducer<TActions> = {
    [K in keyof TActions where TActions[K] extends Promise<any>]: (payload: Unpromise<TActions[K]>)=>void;
    [K in keyof TActions where TActions[K] !extends Promise<any>]: (payload: TActions[K] )=>void;
}

//redux style reducer wireup
function handleActions<TActions>(actions: TActions, reducer: Reducer<TActions>){
...
}

//this would check that MyReducer has methods similar to Actions taking in the resolved promise payload as an argument
handleActions(new Actions(), new MyReducer());