How do I infer generics from an extended interface?

Issue

I have an interface with a generic field, sort of like

interface HasVal<T> {
    val: T
}

I want to create a function like get_val which accepts any HasVal implementer and returns it's val:

function get_val<T, Hv extends HasVal<T>>(hv: Hv): T {
    return hv.val
}

This doesn't work, because T doesn't get narrowed for some reason:

class MyVal {
    val = "hello world!"
}

let u = get_val(new MyVal());
// type of u is unknown :(

Is there a type signature for get_val that will return the proper type without requiring the caller to specify it?

Solution

The problem with

declare function get_val<T, Hv extends HasVal<T>>(hv: Hv): T;

is that generic constraints are not used as inference sites for other generic type arguments. (Such a feature was suggested at microsoft/TypeScript#7234 but eventually declined in favor of other techniques.) So while Hv can be inferred from the hv argument, T cannot be inferred from the Hv extends HasVal<T> constraint. There's pretty much nowhere from which T can be inferred, so it falls back to the implicit unknown constraint, and you get the undesirable behavior.

---

Given this example, I'd be inclined to just remove the Hv type parameter entirely. If you want to infer T from an input of type HasVal<T>, tell the compiler that hv is of type HasVal<T> directly, not indirectly via a constraint:

function get_val<T>(hv: HasVal<T>): T {
    return hv.val
}

And now things behave as desired:

let u = get_val(new MyVal());
// let u: string

T is inferred as string because a MyVal instance is seen as a valid HasVal<string>.

Playground link to code

Answered By - jcalz