Type guard not working in unreachable code

[hraban]

TypeScript Version:

ts@next (Version 3.4.0-dev.20190313)

Search Terms:

typescript
typeguard
type guard
unreachable
undefined
defeat

any permutation thereof

Code

// Please enable TSC option strictNullChecks

function onereturn(y: string | undefined) {
    if (!y) {
        throw new Error();
    }
    // No error: great.
    return y.length;
}

function tworeturn(y: string | undefined) {
    return 3; // temporarily disable function
    if (!y) {
        throw new Error();
    }
    // Why does this now error?
    return y.length;
}

Expected behavior:

Either:

• Consistent type checking regardless of unreachability, or
• a "code unreachable" error.

I don't think the unreachable version should necessarily be error free. I just think the current way of erroring is very unintuitive and confusing.

Actual behavior:

The type guard only works if the code is reachable. Otherwise, it stops working, and the error only shows up when you use the variable further downstream as if the type guard had worked.

Playground Link:

https://www.typescriptlang.org/play/#src=%2F%2F%20Please%20enable%20TSC%20option%20strictNullChecks%0D%0A%0D%0Afunction%20onereturn(y%3A%20string%20%7C%20undefined)%20%7B%0D%0A%20%20%20%20if%20(!y)%20%7B%0D%0A%20%20%20%20%20%20%20%20throw%20new%20Error()%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20%2F%2F%20No%20error%3A%20great.%0D%0A%20%20%20%20return%20y.length%3B%0D%0A%7D%0D%0A%0D%0Afunction%20tworeturn(y%3A%20string%20%7C%20undefined)%20%7B%0D%0A%20%20%20%20return%203%3B%0D%0A%20%20%20%20if%20(!y)%20%7B%0D%0A%20%20%20%20%20%20%20%20throw%20new%20Error()%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20%2F%2F%20Why%20does%20this%20still%20error%3F%0D%0A%20%20%20%20return%20y.length%3B%0D%0A%7D

Related Issues:

Couldn't find anything related in bug tracker.

[RyanCavanaugh]

Duplicate #26914

[hraban]

Ah , sorry about that. Thanks!

…

On Thu, Mar 14, 2019 at 23:05, Ryan Cavanaugh ***@***.***> wrote: Duplicate [#26914](#26914) — You are receiving this because you authored the thread. Reply to this email directly, [view it on GitHub](#30358 (comment)), or [mute the thread](https://github.com/notifications/unsubscribe-auth/AAIafFJHdB4j3LgFrC_oyg64xDP0ZoRdks5vWtW3gaJpZM4bs_oD).