Partial-izing the template does not behave as expected

[ktalebian]

Here is an interface that takes in two templates. the returned function should be a partial of one of the templates (this is a take on the React-Redux):

export type IState<TState, TOwnProps> = {
    connect: (mapStateToProps: MapStateToProps<TState, Partial<TOwnProps>>) => any;
}
export type MapStateToProps<TState, TProps> = (state: TState) => TProps;

Now, here is the example of a state that would want to use this:

interface IAppState {
    services: string[]
}
    
interface IAppProps {
    services: string[];
    count: number;
}
    
const MyState: IState<IAppState, IAppProps>;
MyState.connect(
    (state) => {
        return {
            services: state.services,
            foo: null
        }
    }
);

MyState.connect should expect one function (which it does). The argument should be of type IAppState (which it is). But the return is suppose to be a Partial<IAppProps>. However, as you can see I am returning foo: null too and the service is not complaining about this.

What am I doing wrong here?

[mhegazy]

This is not specific to map types, or Partial. the excess property checks (that should catch foo as an extra property) do not run on return positions of contextually typed functions. #12632 tracks that.