// @flow

interface ExtraField {
  note: string;
}

type Success = ExtraField & { success: true, value: boolean };
type Failed  = { success: false, error: string };

type Response = Success | Failed;

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

Cannot get `response.error` because: Either property `error` is missing in `ExtraField` [1]. Or property `error` is missing in object type [2]

type ExtraField = {
  note: string
}