Type 'T' is not assignable to type 'T extends Sub ? Sub : T'.

[taralx]

Bug Report

🔎 Search Terms

variance, conditional, subtype, limit

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about variance and conditionals.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Base {
    base: unknown;
}

interface Sub extends Base {
    sub: unknown;
}

function transform<T extends Base>(t: T): T extends Sub ? Sub : T {
    return t;
}

🙁 Actual behavior

"Type 'T' is not assignable to type 'T extends Sub ? Sub : T'."

🙂 Expected behavior

No errors. Either T extends Sub, and so is assignable to it, or it doesn't, and so is assignable to itself.

[MartinJohns]

See #48243, #47633 and probably many others.

Resolving of conditional types involving unbound generic type arguments is deferred. The compiler doesn't know what type T extends Sub ? Sub : T resolves to, because it doesn't know what type T is.

[taralx]

Any recommendations on how to express this differently that doesn't run into this problem? Perhaps there is a way to express the join/meet operation without conditionals.

[taralx]

FWIW I tried using [T] extends [Sub] to avoid distributive behavior but it doesn't help.

[whzx5byb]

Any recommendations on how to express this differently that doesn't run into this problem? Perhaps there is a way to express the join/meet operation without conditionals.

interface Base {
    base: unknown;
}

interface Sub extends Base {
    sub: unknown;
}

function transform<T extends Base>(t: T extends Sub ? T & Sub : T): T extends Sub ? Sub : T {
    return t;
}

var t = transform({base: 0, foo: 0})
// ^? var t: { base: number, foo: number }
var sub = transform({base: 0, sub: 0, foo: 0})
//  ^? var sub: Sub

I don't know why T extends Sub ? T & Sub : T is assignable to T extends Sub ? Sub : T since it doesn't meet the conditions mentioned in #46429.
Seems like a black magic 😎

[taralx]

Maybe syntax-driven? Would make sense.

[typescript-bot]

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