reducer.js

// This won't type check even when correct
export default function reducer(state: SessionState = initialState, action: SetCurrentUserAction): SessionState {
  let newState
  switch (action.type) {
    case 'SET_CURRENT_USER':
      newState = { loaded: true, currentUser: action.payload }
  }
  return newState
}

// This type checks even when incorrect
export default function reducer(state: SessionState = initialState, action: SetCurrentUserAction): SessionState {
  switch (action.type) {
    // case 'SET_CURRENT_USER':
    case 'BAD':
      state = { loaded: true, currentUser: action.payload }
  }
  return state
}