🔎 Search Terms

"comment in type cast"

🕗 Version & Regression Information

This is the behavior in every version I tried (nightly 5.6.0-dev.20240711 via playground, 5.5.3 via playground, 5.4.5 in Node.js 18, 5.4.5 in Node.js 20, 4.9.5 via playground), and I reviewed the FAQ for entries about this issue

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.3#code/GYVwdgxgLglg9mABMOcAUBKRBvRAnAUyhDyQBYAmRAXwChaB6BxKACxgGdEATOArsHCgs8MAOZiCeRAENEAFQDKiKXjh4ANI2YAjEMIhwAtgAcYAG34s4iAFLLD3Ai1YzhhYqQ7bEAA3BOwDBgBNy smDciCEAblKIEDLm5ly KHC AIS0oJCwCIjBhniE0JgAXNEgRjrx2LSIjfhEJEhoDU2dTIgcxs6GRkYEYFAdnY3pslyYiAC8AHxVNVJjiBiYtHQ54NDwSMWlUBVLtdL1nR6tiO3jTd29QwnGQyOrnemr6xib9Lm7BUlegB9A4EMoYSpgaqnHCrS6ka6TGTTLALE5SL4-bZ5PayFJwIGAgmg8GQ6F1OEtBE3W7IVBTa6oxZQ5Z4T4bLaGMC9SwAOnMcDEaF8RXUh0qABJsKKSmCjhhqL4MABuWhcnkEfmC4UkqCS7C6zCKlVqhAarVC3xEkFiuX662GhVK1XquB8gWW60O23Qe34wn x3G5VAA

💻 Code

The issue:

function foo() { return 42 }

// this does not trigger a TS error,
// but compiles to JS code that returns
// `undefined` and never calls `foo`!
function incorrect(): number {
    return (
        // some comment
        foo as () => number
    )()
}

Some cases that don't exhibit the issue:

function correct(): number {
    return (
        // some comment
        foo
    )()
}

function also_correct(): number {
    return (foo as () => number)()
}

function also_also_correct(): number {
    return (
        foo as () => number
    )()
}

For reference, the incorrect function above compiles to the following JS code:

function incorrect() {
    return 
    // some comment
    foo();
}

🙁 Actual behavior

The following code:

return (
    // some comment
    foo as () => number
)()

Compiles to:

return
// some comment
foo()

Where the function returns undefined and foo is never called.

🙂 Expected behavior

It should instead compile to something like:

return (
  // some comment
  foo
)()

This happens, I think, because the compiler attempts to remove unnecessary parenthesis around the type cast, which in this situation turn out not to be unnecessary.

Additional information about the issue

The issue does not occur if removeComments is enabled.