Aliasing a type in a conditional type changes the result

Bug Report

🔎 Search Terms

any, alias, conditional type,

🕗 Version & Regression Information

All versions are effected.

⏯ Playground Link

Playground Link

💻 Code

interface User {
    id: number;
    username: string;
}

type Q<T> = {a: keyof T & string };

type t1 = Q<any> extends Q<User> ? true : never;

type qAny = Q<any>;
type t2 = qAny extends Q<User> ? true : never;

🙁 Actual behavior

t1 is true, and t2 is never.

🙂 Expected behavior

Both, t1 and t2 should be the same and be never, because {a: string} extends {a: 'id' | 'username'} is false. The only difference is that the left value of extends is for t2 an alias, but the same type as in t1.

Interesting might be that t2 was in v4.1 true as well, but started to get the right value in v4.2.

Maybe related to https://github.com/microsoft/TypeScript/issues/31295

[tjjfvi]

I believe this boils down to an unsound variance check. TS detects Q<T> as contravariant, since keyof is contravariant, and thus simplifies the check of Q<any> extends Q<User> to User extends any, which is obviously true. However, keyof is not always contravariant; for example:

type A = { x: 1, y?: 2 }
type B = { x: 1 }

const p: A = null! as B; // ok, B is assignable to A
const q: keyof B = null! as keyof A; // error, keyof A is not assignable to keyof B

type Q<T> = { x: keyof T };
type T1 = Q<A> extends Q<B> ? true : never; // true
type QA = Q<A>;
type T2 = QA extends Q<B> ? true : never; // never

T2 is never because the intermediate type alias causes the instantiation of Q<A>, making the extends use a structural comparison instead of the unsound variance comparison.

[andrewbranch]

IIUC #48070, or at least the draft fix that Anders put up, is specific to signatures. I think @tjjfvi nailed it. @ahejlsberg thoughts / triage help?