Incorrent type narrowing when using a generic

[imaliouris]

🔎 Search Terms

• generic type infer

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAggNnAsgewCYTgBQIYEsBOAzgDwAqANFJlBAB7AQB2qhUA1hCMgGZSkB8UALxQA3gCgoUqAG0OIKLkZUAugC4xk6drD5kkfKA3yt2qQDdscAK4QNpOZxWmoAX3GuZmZ+IDGcbEJWAGFsBgBzZHwFCW1fZEZCYHxrX2AogAowawAjOFxfRVQNJPwlcIBKMXd3PwCgvggk4PrWWOl4xOTU9PwMlx1c-MLcYqhS8vIB6Wy8gqhfMIhI6I1QiKiQFyrRGvFxbmtGNNwEqHCIYEw9A1BiAGkaeiYWdk4eRubW-gzRKF19BBDCBKJYbNBXBp4Eg0BgcAQSKQmsAWoFCJR7vwKvZkaigjJ7ipNNpcLwsjcgaBhEIRAByRYbaK0nbTKT4S7WfDKMG2KAAej5qGQTUYtOAUHZhGQcHM0GAAAswlAedBcKxQJAoB91stNlBsMwoAq1YpWIxkOKFdA6JA0hBUFAchBFeZTpyXLVtOzgJzuVZbB59p0kgswgBGYTnS7XQHA34A24gDQAIgZuuiydB-rsUEYEAA7lAdSsQBlaWHma4qgLFOLJdLZerFZb5dBvb6jeBVSEliX9odjsBTsoLlcKcCAEy-f7j0BZ8FuKEIFDoLB4IhkXGtSjyD5Ir5orE4g-43e8fco1pE9pSUlQcmxqk0ulpkvM4lmNkcrnK7P8vkSk0DZNEaza-guJoatA2q9nqBoOsarCQa2Ty2gwDpOi6br4B6+xet+frgoGfgJCGDITpGo4xomU6iAmlJJlAqawRm862BoeaFsWmxlhWFRVv+tZQEKIpioBUoynKYFWoBPo-lBppFixCjwaBYS0k21q0Gh9qOs62Cusg7pAA

💻 Code

type AllModelPairs<T, P extends keyof T> = {
    [key in P]: {
        property: key
        value: T[key]
    }
}[P]

class Category {
    constructor(public id: string) {}
}

class TestClass {
    constructor(
        public id: string,
        public category: Category
    ) {}
}

function getProperty<K extends keyof TestClass>({ property, value }: AllModelPairs<TestClass, K>): TestClass[K] {
    if (property === 'category') {
        return value //doesn't resolve that value is type of Category and this is not the expected behaviour
    }

    return value
}

const cat1 = getProperty({property: "category", value: new Category('1')}) // it resolves that the return type is Category

function getProperty2({ property, value }: AllModelPairs<TestClass, keyof TestClass>): TestClass[keyof TestClass] {
    if (property === 'category') {
        return value // resolves that value is type of Category and this is the expected behaviour
    }

    return value
}

const cat2 = getProperty2({property: "category", value: new Category('1')}) // it doesn't resolve that the return type is Category and that's the expected behaviour

🙁 Actual behavior

Typescript doesn't infer that value is type of Category when property is 'category' when property is a generic

🙂 Expected behavior

Typescript should infer that value is type of Category when property is 'category'

Additional information about the issue

No response

[jcalz]

"This is a crash" No, that means the compiler process itself terminated unexpectedly.

This is a known limitation or missing feature. Control flow-based case analysis and generics do not play nicely together. If you use generics, then control flow does not currently do anything to the type parameter: see #33014, #27808, for example.

[typescript-bot]

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