[Suggestion] Mapped types with inner generic arguments and constraints

[richardspence]

Proposal

I'd like to suggest a feature where in mapped types we could unwrap some of the properties. I'm trying to solve a problem where I want to unpromise a type, and this requires access to the inner generic argument type. This is useful in creating redux style reducers that are strongly typed.

Example

//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;
}

Complete Example

//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());

related to #12424

[mhegazy]

See #12424 and #17077

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.