title: CombineReducers with Typescript returns error "Argument of type is not assignable to parameter of type 'ReducersMapObject'"
body: <p>I'm trying to setup a front-end environment with react + redux + typescript, but I am struggling to get it work with combineReducers.
  I get an error: Argument of type is not assignable to parameter of type 'ReducersMapObject'. See the full error message below the code.</p>
  
  <p>STATE: (types/index.tsx)</p>
  
  <pre><code>export namespace StoreState {
  
    export type Enthusiasm = {
      languageName: string;
      enthusiasmLevel: number;
    }
  
    export type All = {
      enthusiasm: Enthusiasm
    }
  }
  </code></pre>
  
  <p>STORE: (store.tsx)</p>
  
  <pre><code>import { createStore } from 'redux';
  import reducers from './reducers/index';
  import { StoreState } from './types/index';
  
  let devtools: any = window['devToolsExtension'] ? window['devToolsExtension']() : (f:any)=&gt;f;
  
  const Store = createStore&lt;StoreState.All&gt;(reducers, devtools);
  export default Store;
  </code></pre>
  
  <p>REDUCER: (/reducers/HelloReducer.tsx)</p>
  
  <pre><code>    import { EnthusiasmAction } from '../actions';
      import { StoreState } from '../types/index';
      import { INCREMENT_ENTHUSIASM, DECREMENT_ENTHUSIASM } from '../constants/index';
  
      export const enthusiasm = (state: StoreState.Enthusiasm, 
  action: EnthusiasmAction): StoreState.Enthusiasm =&gt; {
        switch (action.type) {
          case INCREMENT_ENTHUSIASM:
              return { ...state, enthusiasmLevel: state.enthusiasmLevel + 1 };
          case DECREMENT_ENTHUSIASM:
              return { ...state, enthusiasmLevel: Math.max(1, state.enthusiasmLevel - 1) };
          default:
              return state;
       }
     }
  </code></pre>
  
  <p>COMBINE REDUCERS (/reducers/index.tsx)</p>
  
  <pre><code>import { StoreState } from '../types/index';
  import * as enthusiasmReducer from './HelloReducer';
  
  import { combineReducers } from 'redux';
  
  const reducer = combineReducers&lt;StoreState.All&gt;({
      enthusiasm: enthusiasmReducer
  });
  
  export default reducer;
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/6Jk7o.png" rel="noreferrer"><img src="https://i.stack.imgur.com/6Jk7o.png" alt="enter image description here"></a></p>