type MaybeResult<T> = T | Error; // ここはプロパティ式にしてもいいしなんでも良い

const isError = <T>(result: MaybeResult<T>): result is Error => result
instanceof Error;

const func = (flag: boolean): MaybeResult<string> => (flag ? "success"
: Error("error"));

const maybeResult = func(true);
if (isError(maybeResult)) {
  // 場合に応じたエラー処理
  const b = maybeResult;
  return;
}

// 通常の処理
const a: string = maybeResult;