// NOTE: without the ReducerState indirection, TypeScript would reduce S to be the most common
    // supertype between the reducer's return type and the initialState (or the initializer's return type),
    // which would prevent autocompletion from ever working.

    // TODO: double-check if this weird overload logic is necessary. It is possible it's either a bug
    // in older versions, or a regression in newer versions of the typescript completion service.
    function useReducer<R extends Reducer<any, any>>(
        reducer: R,
        initialState: ReducerState<R>,
        initializer?: undefined
    ): [ReducerState<R>, Dispatch<ReducerAction<R>>];

  const [state, dispatch] = useReducer(
    (state, payload: number): {count: number} => ({
      ...state,
      count: state.count + payload,
      extraProp: 'asdf', // Type '{ count: number; extraProp: string; }' is not assignable to type '{ count: number; }'. Object literal may only specify known properties, and 'extraProp' does not exist in type '{ count: number; }'.(2322)
    }),
    { count: 0 }
  );