False "variable is used before being assigned" error

[sempasha]

Bug Report

🔎 Search Terms

Variable is used before being assigned ts(2454)

🕗 Version & Regression Information

According to TypeScript Playground error appears for TypeScript versions >= 3.3.3 && <= 4.5.4 and for latest nightly 4.6.4-dev.20220112. For versions ``, 3.5.1 and `3.6.3` no error happens.

⏯ Playground Link

Playground link with relevant code

💻 Code

let value: 'fail' | 'ok';
try {
  value = 'ok';
} catch (error) {
  value = 'fail';
} finally {
  console.log(value);
}

🙁 Actual behavior

Error Variable 'status' is used before being assigned.ts(2454) at finally block.

🙂 Expected behavior

Expect provided code sample has no error at finally block, because variable status will be surely initialized.

[sempasha]

Workaround is to initialize status variable immediately

let value: 'fail' | 'ok' = 'ok';
try {
  // ...
} catch (error) {
  value = 'fail';
} finally {
  console.log(value);
}

[RyanCavanaugh]

We don't have the necessary machinery to figure this out. Determining that this is OK requires knowing that the line status = 'fail'; cannot throw, and we don't track what kinds of statements/expressions can/can't throw. Even if we did, an assignment statement can throw due to the target being in TDZ.

[typescript-bot]

This issue has been marked as "Won't Fix" and has seen no recent activity. It has been automatically closed for house-keeping purposes.