字面量推断 Literal Inference

当变量初始化一个字面量对象时，对象的类型会被推断出来

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解决：

type Method = "GET" | "POST"
declare function handleRequest(url: string, method: Method): void

const req = { url: "https://example.com", method: "GET" }

handleRequest(req.url, req.method as Method)