// @flow

type Success = { value: boolean };
type Failed  = { error: string };

type Response = Success | Failed;

function handleResponse(response: Response) {
  if (response.value) {
    var value = response.value;
  } else {
    var error = response.error; // Error!
  }
}

Cannot get `response.error` because property `error` is missing in `Success` [1].