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?

まず、提供された async の例を再現し、その到達可能性を直接的な returnNever() の例と比較します。await と Promise に関するコンパイラーの control-flow、reachability、definite-assignment の解析を読みます。完了条件は、その動作が意図されたものかどうかを判断すること、または期待される結果に対する焦点を絞った回帰テストを追加することです。