type guard does not analyze correctly when in an unreachable code block and allowUnreachableCode is true

[thomasrea0113]

🔎 Search Terms

type guard unreachable code allowUnreachableCode

🕗 Version & Regression Information

Version 5.3.3

⏯ Playground Link

https://www.typescriptlang.org/play?allowUnreachableCode=true&ts=5.4.0-dev.20240215#code/PTAEHUFMBsGMHsC2lQBd5oBYoCoE8AHSAZVgCcBLA1UABWgEM8BzM+AVwDsATAGiwoBnUENANQAd0gAjQRVSQAUCEmYKsTKGYUAbpGF4OY0BoadYKdJMoL+gzAzIoz3UNEiPOofEVKVqAHSKymAAmkYI7NCuqGqcANag8ABmIjQUXrFOKBJMggBcIYqohCgAgqAAvKAA3oqgDWL5oIKolJzMigC+wSVEoABCVaAVAGS19Y3Sza3tzADc3cEInK0igkPVABQMzWUAlM3iopsAfGilKaA7YsJmePsB0lWV1QBEsxnMb8vwqzQSeSYACqnCcDFM0ncwx2e32VVOijqjREqS2bXYkHhTlQ7DInEWkwaK0E8HcAWg8GYWze2GglMk8DI0Te+2CKIoaKEAx2+zZKJRJLJkApVJ2TzZPUUJIBQI4qFB4Mh0O2uxG8MqiORjSF5Mp1NpMAZEiZLLZRNR125vP5AuJf1JerFDAlSwaQA

💻 Code

// Welcome to the TypeScript Playground, this is a website
// which gives you a chance to write, share and learn TypeScript.

// You could think of it in three ways:
//
type A = {
    a: string
}

type B = A & {
    b: string;
}

const isB = (a: A): a is B => typeof (a as any).b === "string"

const withUnreachable = (a: A) =>
{
    if (true) return;

    console.log("hello world")

    if (isB(a))
        console.log(a.b)
}

const withoutUnreachable = (a: A) =>
{
    console.log("hello world")

    if (isB(a))
        console.log(a.b)
}
   

🙁 Actual behavior

type error is thrown because the type guard is not executed

🙂 Expected behavior

type guards should be processed even if the code is unreachable, or all type checking should be disable for unreachable code

Additional information about the issue

I tested with all versions available on the playground, none work. Full type checking is not executed on unreachable code even when allowUnreachableCode is true. It seems that the type guards do not infer the guarded type if it is in an unreachable block, so I get a compile error. This isn't a huge deal but during testing sometimes it's easier to force an early return.

[fatcerberus]

Duplicate #26914

[typescript-bot]

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