Reducer issue

reducer.ts

import { AnyAction, Reducer } from 'redux'

type UserState = Readonly<{
  id: number
}>

export const initialState: UserState = {
  id: -1
}

const userReducer: Reducer<UserState> = (state = initialState, action: any) => {
  return {
    id: 0,
    foo: 'bar' // Why doesn't this raise an error?
  }
}

const userReducer2: Reducer<UserState> = (state = initialState, action: any): UserState => {
  return {
    id: 0,
    foo: 'bar' //this raises an issue due to explicitly defining the return type.
  }
}