interface State {
  a: string;
  b: boolean
}

const initialState: State = { a: '', b: false }

class SetA extends ActionStandard<string> {}
class SetB extends ActionStandard<boolean> {}

function stateReducer(state =  initialState, action: ActionStandard): State {
  switch (action.type) {
    case SetA.type:
      return { ...state, b: action.payload }; // assigning boolean to string, no errors
    case SetB.type:
      return { ...state, a: action.payload }; // assigning string to boolean, no errors
    default:
      return state;
  }
}

const A_TYPE = 'A';
const B_TYPE = 'B';

class SetA extends Action {
  readonly type = A_TYPE;
  payload: string;
}
class SetB extends Action {
  readonly type = B_TYPE;
  payload: boolean;
}

type MyActions = SetA | SetB;

function stateReducer(state =  initialState, action: MyActions): State {
  switch (action.type) {
    case A_TYPE:
      return { ...state, b: action.payload }; // assigning boolean to string, error
    case B_TYPE:
      return { ...state, a: action.payload }; // assigning string to boolean, error
    default:
      return state;
  }
}