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

[nikeee]

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: #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?

[andrewbranch]

I think this makes sense, though two things are unclear to me:

• how complex/expensive this would be to implement
• if this comes up in real-world scenarios

Have you found real cases where you have a Promise<never>? Would love to see some motivating examples.

[nikeee]

Consider a logErrorAsync function that returns Promise<never>. It awaits something and then throws an error on every path (or goes into an infinite loop etc.). If the function does not return a Promise<never> but a never instead, it behaves as expected.

As of TS 3.7.3:

declare function fetchB(): Promise<string>;

async function logErrorAsync(err: any): Promise<never> {
 await fetch("/foo/...");
 console.log("Error has been submitted to analytics tool");
 throw new Error(err);
}

function logError(err: any): never {
 console.error(err);
 throw new Error(err);
}

(async () => {
 let b: string;
 try {
 b = await fetchB();
 } catch (err) {
 await logErrorAsync(err); // awaiting Promise<never>
 }
 b.toUpperCase(); // "Error: "b" is used before assignment;" would require workarounds
})();

//...
(async () => {
 let b: string;
 try {
 b = await fetchB();
 } catch (err) {
 logError(err); // returns never
 }
 b.toUpperCase(); // No error, as expected
})();

[andrewbranch]

Yeah, since we actually error on unreachable code, this does seem worthy of the bug label.