Union type in element access can prevent deprecated warning from triggering

TypeScript Version: 4.1.0-dev.20201027

Search Terms: Deprecated tag, union, element access

Code

const foo = {
    faff() { },

    /** @deprecated */
    fluff() { }

}

declare const k: "faff" | "fluff";

//Expected: deprecation warning
//Actual: no warning
foo?.[k]?.();

const bar = {
    /** @deprecated */
    faff() { },

    fluff() { }
}

//Expected: deprecation warning
//Actual: deprecation warning
bar?.[k]?.();

Expected behavior:

foo?.[k]?.(); should show deprecated warning because if k is "fluff", then we'll be calling a deprecated method.

Actual behavior:

foo?.[k]?.(); has no deprecated warning.

Playground Link: Playground

Related Issues: Couldn't find any

从链接的 TypeScript Playground 复现开始，使用报告中的 TypeScript 版本比较 foo?.[k]?.() 和 bar?.[k]?.() 两种情况。追踪带有 union key 的可选元素访问是如何生成弃用警告的；当 foo 情况报告弃用警告且不导致现有 bar 情况回归时，即表示完成。