The ?: inference failed; no selection was made between ifTrue and ifFalse.

[LiST-GIT]

🔎 Search Terms

?:

🕗 Version & Regression Information

v5.4.5

⏯ Playground Link

https://www.typescriptlang.org/play?noUncheckedIndexedAccess=true#code/MYGwhgzhAECC0G8BQ1XQgFzBglsaAogG4CmAdhgCoCeADiRAFyIppsDkJpFAcmALYl2zANoBGADTQATAF0JrVAF8A3EkXQAJiVBgATiWgj2sWO1nMMdEgHsAZnDUaMDDAB4A0tBIAPF2U0YWCkAMW8-ckDoD2NTc2NiciprCHMpSnD-KIBrEmp7aBCAPgAKDTYMAAscJmiFNgrrZkp6htQwZhCRSllMyJgwMmoRXoB+aDIAV34AIxI9aGZMPRwyAHNoAHpN6EpKw1W7efJgQ32DaAB3SAmbDHRJ4FOoO0mQFWgqwyt6dEqbN6aaBzLQkFx6firMAzECGG4kHBfBZgCbTOYLGzI9AYFbrAB0GgAlJ1ur0ENADBhJnoyKiQCBoDdBtQPkp1GwIJEACLYMAlQksNqoIj6RnQAC8n2qEDxLkwJU43AwfEE7Ck7Gk7EJH220AAyjjVhsard7mAniRaFgYSQCUKxZLxFI5Fsdl0etATRBHs8IK96dRPWQjnoDJo7UKRQsZhKpTVZa4FVwkiqhM7ta7oDw0fNPTAyHdGRardDYRG2jHHdIpABmXq6929L0+hh+t4gQOHeZh8vQNlKIA

💻 Code

class A {
    static EventTypes: {
        'eventName': [1, 2],
    };

    declare ['AA']: typeof A;

    test<K extends A, F extends K['AA']['EventTypes'], T extends keyof F>(
        this: K,
        type: T,
        a: F[T] extends any[] ? number : string // The inference here was not successful; the type should be determinable as either a number or a string.
    ): F[T] { return null as any; }

    sendData() {
        var a = this.test('eventName', '2'); // String is not acceptable.
        a = [1, 2] // F[T] is successfully inferred.
        var b = this.test('eventName', 2); // Number is not acceptable.
        b = [2, 3] // F[T] is successfully inferred.
    }
}

🙁 Actual behavior

Argument of type 'string' is not assignable to parameter of type 'this["AA"]["EventTypes"]["TCP:SEND_DATA"] extends any[] ? number : string'.
Argument of type 'number' is not assignable to parameter of type 'this["AA"]["EventTypes"]["TCP:SEND_DATA"] extends any[] ? number : string'.

Using the ?: operator, neither ifTrue nor ifFalse was selected. I am unsure of what type to pass in to satisfy the conditions.

🙂 Expected behavior

I believe the ?: operator should never fail; it should always select and return one of the options.
Of course, in my test, I hope it returns a number.

Additional information about the issue

No response

[MartinJohns]

I believe the ?: operator should never fail; it should always select and return one of the options.

It's not failing, your type simply can't be resolved because the type of this is not known. It could be very well a subclass with a more constrained property. Such is the limitation with conditional types. When dealing with conditional types all involved types must be known, and what type this actually is is not known.

See also #51288 and other issues regarding this and conditional types.