Inheritance-based generic type inference in Typescript

Take a look at this piece of code:

class A<T> { t?: T; }
interface B {}
class C implements A<B> {}
function f<T1 extends A<T2>, T2>(a: T1): T2 | undefined { return a.t; }

const result = f(new C());
const result2 = f(new A<B>());

Interestingly, the type of result and even result2 will be unknown, despite being inferable that it should be B based on context since C is implementing A<B>.

Why doesn't TypeScript automatically infer this? Is it due to a missing feature, an unsound inference, or is there an alternative way to achieve the desired behavior?

typescripttype-inference

Answer №1

After digging deep, I discovered that TypeScript is truly exceptional!

class X<Y> { y?: Y; }
interface Z {}
class W extends X<Z> {}

type FindValue<V> = V extends X<infer U> ? U : never;
function findValue<V1 extends X<V2>, V2 = FindValue<V1>>(b: V1): V2 | undefined { return b.y; }

const outcome = findValue(new W());
const outcome2 = findValue(new X<Z>());

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