Generic properties are not correctly inferred

[markostanimirovic]

🔎 Search Terms

"generic type inference"

🕗 Version & Regression Information

TS v5.3.3

It's not a regression.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAysBOBLAdgcwArwPZgM4B4BRZYRUAPigF4oBvAKCigG0BpKFKAawhCwDMoxUqCgBDXEJJkQbALpQIAD2ARkAE0m4EKVFAD8UdgC4oyCADcI8OaeEz5AbnoBfZ-VCRYOtJhyteImkKam5eAW8kX2w8IJEQcnd+AFdkAGNSLGQoABssVACQInIACh4QUzgojBjC4oBKOkYoNKzcLByIADo81DLeeucXenoU9Mzs62x4IkUVNU06KGTkxHVTbWqoF1LGhiZeusJSgHIVtZPB13ogA

💻 Code

type StringProps<Entity> = {
  [K in keyof Entity as Entity[K] extends string ? K : never]: Entity[K];
};

type StringPropKey<Entity> = keyof StringProps<Entity>;

function logKey<E>(key: StringPropKey<E>) {
  console.log(key);
}

function error<E extends { uuid: string }>() {
  logKey<E>('uuid');
}

🙁 Actual behavior

Compilation error: Argument of type 'string' is not assignable to parameter of type 'keyof StringProps<E>'. Type '"uuid"' is not assignable to type '(E[K] extends string ? K : never) | (E["uuid"] extends string ? "uuid" : never)'

🙂 Expected behavior

There should not be a compilation error. As a workaround, we can explicitly check if E extends { uuid: string }:

function error<E extends { uuid: string }>() {
  // this
  logKey<E extends { uuid: string } ? E : never>('uuid');
}

but this should not be required because E already extends { uuid: string }.

Additional information about the issue

No response

[jcalz]

Seems like you want #48992; TS doesn't have the kind of abstract reasoning necessary to understand what StringProps<E> does for generic/arbitrary E.

[markostanimirovic]

closing as duplicate