Well, the forFeature() method is a generic as well, and we can specify this one explicitly like so:

// counter.module.ts

const CounterState = {
  data: number
};

const reducers: ActionReducerMap<CounterState> = {
  data: counterReducer
}

@NgModule({
  imports: [
    StoreModule.forFeature<CounterState, Action>('counter', reducers)
  ]
})

This protects us from adding a state mapping object that it does not expect to the forFeature() method. For instance, the following would render an error:

// example of what NOT to do

interface State {
  test: string;
}

function testReducer(state ="", action: Action) {
  switch(action.type) {
    default:
      return state;
  }
}

const reducers: ActionReducerMap<State> = {
  test: testReducer
};

@NgModule({
  imports: [
    BrowserModule,
    StoreModule.forFeature<CounterState, Action>('counter', reducers)
  ],
  exports: [CounterComponent, CounterListComponent],
  declarations: [CounterComponent, CounterListComponent],
  providers: [],
})
export class CounterModule { }

The reason for this is that we are providing the wrong type to the forFeature() method. It expects a reducer parameter to be something of type ActionReducerMap<CounterState>, which it clearly is not, as we are sending in ActionReducerMap<State>.