Type Prediction for union with typeguard looks different from when we do not use typegurad

[ryoppippi]

🔎 Search Terms

TypeGuard
Union

🕗 Version & Regression Information

Works After TS 4.6

⏯ Playground Link

https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABDAzgORAG0wCgB4Bci4A1mHAO5gCUReyKiYWmA3gFCJeIBOAplBA8k9ALzimLANzsAvu3ZQAngAc+iAKI8ecHgFUw8MAB4AKgBpEexKM3bdAPhuIA2nsvgAJn2AwwfTwBdRAAfV2ZsS1NAhXZQSFgERCg+FCgcAEMeAHMiLR19QwRjNJ4-bKdqDm5ECAQ01z5tSwA3DMwQPmDbLOzObhhgHABCVAxsHCaeair+mq4oAAsdCkQpmRr5Gv5BYUQ2jvVEAHpjxFLy0OIwb19-TzkFeOgjZNSoACZMnLz7QqMSlAymAKohZjU6mAGi4pq12p1uohenNkEMpohhhIIphwfMFstKGttBtuFtuDshEgDp0TmcLiC5EA

💻 Code

function isNull(x: unknown): x is null{
    return x === null;
}

type ErrorUnion<T, U = Error> = [U, undefined] | [null, T]


function test(arg: ErrorUnion<string> ){
    const [err, value] = arg
    if(!isNull(err)){
        throw err;
    }
    return value  // string | undefined
}

function test2(arg: ErrorUnion<string> ){
    const [err, value] = arg
    if(err !== null){
        throw err;
    }
    return value // string
}

🙁 Actual behavior

The value variables in test and test2 are predicted as different types

🙂 Expected behavior

Both value in test and test2 should be predicted as string

Additional information about the issue

No response

[fatcerberus]

Looks like a duplicate of #55766 (comment).

[ryoppippi]

@fatcerberus
Yeah looks like duplicated one
Thanks

[MartinJohns]

What does it mean if it seems wired?

[fatcerberus]

What does it mean if it seems wired?

Probably that the programmer had too much coffee.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[ryoppippi]

Why you guys are so offensive.....