Description
π Search Terms
reachability never
π Version & Regression Information
It's observed in all TS versions.
β― Playground Link
π» Code
function* never(): Generator<unknown,never> {
throw 'never';
}
function* main() {
try {
yield* never();
} finally {
console.log('finally');
}
console.log('not reachable');
}
π Actual behavior
No error reported.
π Expected behavior
There should be an error Unreachable code detected.(7027)
at line 11 console.log('not reachable');
.
Additional information about the issue
Reachability analysis is working for normal function: https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABGApgNxQJwBQEoBcy6WiA3gFCKJQAWmcA7ogOSoabMDc5AvueaEiwEiALYBDGGDxlK1TAE9ZVKmyx5uVHomBTxAG31KKKxBAQBnOPpQA6fXADm2ZrrAGjzXJsR8q5sCsbeycXMDgoREwUcQgacQAjGy9uPiA
It's broken for Promise and we have another issue tracking that one: #34955
Generator functions are being used as coroutine in libraries like co.js, Effect.js, redux-saga, Effection, and more. It's common to write a coroutine operator that suspend the thread forever, thus taking function never(): Generator<Instruction, never>
signature. It would benefit users if the TypeScript control analysis can infer unreachability after yield* never()
.