Possible bug with undefined narrowing via type guards with conditions

[ngouy]

Bug Report

🔎 Search Terms

Typeguard narrowing undefined conditions

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type guards

⏯ Playground Link

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type rankedObject = {
    rank: number | undefined
}

const foo: rankedObject[] = [
    {rank: 1}, {rank: 4}, {rank: -1}, {rank: undefined}
]

const sortRanks = (element1: rankedObject, element2: rankedObject) => {
    if (element1.rank !== element2.rank) {
        if(element1.rank && !element2.rank) return -1;
        if(!element1.rank && element2.rank) return 1;
        // error here saying: element1|2 is possibily undefined
        return element1.rank - element2.rank;
    }
}

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Workbench Repro

💻 Code

type rankedObject = {
    rank: number | undefined
}

const foo: rankedObject[] = [
    {rank: 1}, {rank: 4}, {rank: -1}, {rank: undefined}
]

const sortRanks = (element1: rankedObject, element2: rankedObject) => {
    if (element1.rank !== element2.rank) {
        if(element1.rank && !element2.rank) return -1;
        if(!element1.rank && element2.rank) return 1;
        // error here saying: element1|2 is possibily undefined, which is not possible
        return element1.rank - element2.rank;
    }
}

🙁 Actual behavior

I have an error saying element1.rank and element2.rank could be undefined
Which is not possible given:
1 - they are not the same
2 - we rulled out (!A && B) and (A && !B)
so the only possibility that left is that A and B are both not undefined

🙂 Expected behavior

no error, given rank are both defined at this point

[fatcerberus]

That this doesn’t work falls under the same reasoning as #52822 (comment). tsc is not a proof solver.

[MartinJohns]

Duplicate of #37258.

[ngouy]

My bad
Anyway I kinda forgot that the ! was ruling out not only undefined stuff, but also falsy stuff
Sorry

[ngouy]

Thank you for your help @MartinJohns and @fatcerberus