TypeScript: 返回对象类型静态推导问题?

Question

有大佬帮忙解释一下为啥return1 的静态推导是下面这样子的嘛

const return1: () => {
    type: string;
    result: number;
    error?: undefined;
} | {
    type: string;
    error: Error;
    result?: undefined;
}