The extends infer for function generics fails.

[vipcxj]

🔎 Search Terms

function generics infer

🕗 Version & Regression Information

all version

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.3#code/PTAEHUFMBsGMHsC2lQBd5oBYoCoE8AHSAZVgCcBLA1UABWgEM8BzM+AVwDsATAGiwoBnUENANQAd0gAjQRVSQAUCEmYKsTKGYUAbpGF4OY0BoadYKdJMoL+gzAzIoz3UNEiPOofEVKVqAHSKymAAmkYI7NCuqGqcANag8ABmIjQUXrFOKBJMggBcISGgoAC0oACCbvCwDKgU8JkY7p7ehCTkVDQS2E6gnPCxGcwmZqDSTgzxxWWVoASMFmgYkAAeRJTInN3ymj4d-jSCeNsMq-wuoPaOltigAKoASgAywhK7SbGQZIIz5VWCFzSeCrZagNYbChbHaxUDcCjJZLfSDbExIAgUdxkUBIursJzCFJtXydajBZJcWD1Ro4zgAHhwAEZwasFDxhJNuI1oHhQABtQSoSicZj8TjsRDSb4AXQAfAAKACU+W8zIA3ooSiVUO1QABhUAAXlVLLZ3A5Hi5nB5-MFwtF-QlUrI0tAAH40GR2CgVckGNBBJAANyarUITiCkwqg3GoXeoOgFT7UAAcjjkBTImEAxoDEEcmYnAY0ncYJ1RFTepToZK4cjsAATNGjTj-YGE0ndSm-QGM1n+oMxPmKIXi6WrOWUCmqzXQE5UPivOLoNAh6AuPEBhIvHnVSGAL7kynUrzJTgNhnMtZm4R24b8HAN00o82OyXfBWK0AarVoiM0VJjTPOk+SZB8GzlJUQ1-Sd9WZY0mWfdkriFYZ3U9b1QF9Ntg1nOsaFgRlo3gjDg0TMBkzTL0+1EHMhwLIsS0sDBYOnRlqxg3U9SfWN2iJVJrxfC0GCtG1QMZcDXQ9dMsNbXtoLDRp6ybfUeNIg8j3ME9aQAZgZJDX05bleQFVCRTFJ0ZQVVAVRwL8f0U-8cRbYCcE-BTtS4kjJ34gzhBwdCZOw+S8KUgiiLglt0w7Ciu3TTNaMHPMGLHZi0C7PV2MUQ8gA

💻 Code

function fn<T1 extends readonly [string, number]>(): T1 {
    type C = T1 extends readonly [string, number] ? true : false;
    const c: C = true; // Type 'true' is not assignable to type 'C'
    const c2: C = false; // Type 'false' is not assignable to type 'C'
    return null as unknown as T1;
}

function fn2<T1 extends string, T2 extends number>() {
    const f = fn<[T1, T2]>();
    type C1 = T1 extends string ? true : false;
    const c1: C1 = true; // Type 'true' is not assignable to type 'C1'
    type C2 = typeof f extends readonly [T1, T2] ? true : false;
    const c2: C2 = true;
}

function fn3<T extends readonly [string, number]>(t: T) {
    const f = fn<T>();
    type C1 = typeof f extends T ? true : false;
    const c1: C1 = true; // Type 'true' is not assignable to type 'C1'
}

🙁 Actual behavior

In a generic function, even if I add restrictions to the generic type T, the function seems completely unaware of this, and the extends test will never pass.

🙂 Expected behavior

C in fun should be true
C1 in fun2 should be true
C1 in fun3 should be true

Additional information about the issue

No response

[MartinJohns]

This is working as intended / a design limitation. Resolving of conditional types involving unbound generic types is deferred. The compiler doesn't know what type T1 is, so it can't resolve the type T1 extends string ? true : false.

Also duplicate of #23132.