Why doesn't awaiting a Promise<never> change reachability?

TypeScript Version: 3.7.2

Search Terms:

• "Promise"
• await
• reachability analysis
• control flow analysis
• definite assignment analysis

Code

(async () => {
    let b: string;
    let a1 = await Promise.reject(); // returns Promise<never>
    b = ""; // Not unreachable?
    b.toUpperCase(); // Not unreachable?
})();

Expected behavior:
As a1 is inferred to be never (e.g. behaves like in the non-promised version), I expected the rest of the code to be marked as unreachable aswell:

function returnNever(): never { throw new Error(); }

(async () => {
    let b: string;
    let a0 = returnNever(); // a0 is never
    b = ""; // Unreachable
    b.toUpperCase(); // Unreachable
})();

Actual behavior:
The code after the never-returning promise is marked as reachable.

Related Question on StackOverflow: Has more code: https://stackoverflow.com/questions/58732814
Related Issue: https://github.com/microsoft/TypeScript/issues/10973 (although marked as "Working as intended", it was changed later. 3.7.2 behaves like the issue opener expected).

If this is not a bug, what is the background for this behavior?

首先复现所提供的 async 示例，并将其可达性与直接的 returnNever() 示例进行比较。阅读 compiler 中围绕 await 和 Promise 的 control-flow、reachability 和 definite-assignment 分析；完成标准是确定该行为是否有意为之，或为预期结果添加一个针对性的回归测试。