Create a root reducer with typesafe-actions

I'm using typesafe-actions and would like to create a root reducer that will allow me to handle global actions like LOGGED_OUT, in order to clear state. Like in the answer to this question.

I've tried a few things but I keep losing the type safety that typesafe-actions provides, and the reducer reverts to an any type.

I have a pretty vanilla type safe actions setup. I haven't posted the permutation solutions of implicit typing that I've tried, because I don't think it will add any clarity to the question.

The closest to a solution being found I could find was in this thread. The maintainer of type safe actions himself posted a response and mentioned that he already had a solution, though from that point on there were no links or references provided.

Any help would be great.

For the record, I can get this to functionally work following the pattern as per the Stack overflow question I posted, however the types are broken.

Turns out I already solved this 9 months ago on a separate project and forgot.

import { combineReducers } from 'redux';
import auth from '../modules/auth/reducer';
import feed from '../modules/feed/reducer';
import post from '../modules/post/reducer';
import profile from '../modules/profile/reducer';
import { StateType, Reducer, RootAction } from 'typesafe-actions';
import { signOut } from 'modules/auth/actions';

const appReducer = combineReducers({
  auth,
  feed,
  post,
  profile
});

type RootState = StateType<typeof appReducer>;

const clearOnSignOutReducer: Reducer<RootState, RootAction> = (
  state,
  action
) => {
  if (action.type === signOut().type) {
    state = undefined;
  }
  return appReducer(state, action);
};

export default clearOnSignOutReducer;