array in conditional type issue

[marc0n3]

🔎 Search Terms

array in conditional type issue inheritance accessor function

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play?#code/MYGwhgzhAECSAuBTAtgbwFDS9AdmZiAhAFwTwBOAljgOboC+6YARmeWMPNKJDAigGEA9jnhhqichmzRKSZBBJx5AbQC6DdOh5RoAEXYB3SdEQAPJDgAmfecNHick6dgiUAXkWI4ArsmaSmlpa8ACeAA6I0ACC5OyhsDhO5AAqEYgAPDEAfNAAvFjRphaI1jAAFNQAZibVkuSIVgCU6gD8dXGNxNEA3MHa4LoAsmB4NJIZKcWWNsqCImIS5NkuWMijYOPlQswAVoicxClNqzLQYZHQVDQAFvBplwWx8YnJD5n8yPaLySoARHIUBA-mpsn0ztgAPSQiEAPVamAh53S0EM5BENHe+WeYASSUk70m-0BChBYMREOh2HhFOgjHoQA

💻 Code

class Item{
    name!:string
}
abstract class ItemContainer{
    items!: Item[]
}

class Drawer extends ItemContainer{
    size!:number
}




type ArrayInnerType< A> =  A extends (infer inferred)[]?inferred:A;



class Manager<T extends ItemContainer>{
    manage(object:T){
        type rightType = ArrayInnerType<ItemContainer["items"]>;
        //      ^?
        type wrongType =ArrayInnerType<T["items"]>;
        //   ^?
    }
}

🙁 Actual behavior

actual type of wrongType is (inferred & inferred[]) | T["items"]

🙂 Expected behavior

wrongType is Item

Additional information about the issue

No response

[MartinJohns]

This is a design limitation. Resolving of conditional types involving unbound generic types is deferred. The compiler does not know what type T is, so it can't know what the conditional type resolves to.

[marc0n3]

Thanks!
cannot the fact that a base class for T (ItemContainer) is defined help in getting at least base class property types?

[RyanCavanaugh]

It'd be wrong to produce Item here since T might be instantiated with a class with a more-specific type in that field, e.g.

class Item {
    name!: string
}
class SuperItem extends Item {
    isSuper!: true;
}
abstract class ItemContainer {
    items!: Item[]
}
class SuperItemContainer extends ItemContainer {
    items!: SuperItem[];
}

class Drawer extends ItemContainer {
    size!: number
}

type ArrayInnerType<A> = A extends (infer inferred)[] ? inferred : A;

class Manager<T extends ItemContainer> {
    manage(object: T): ArrayInnerType<T["items"]> {
        // Alleged: Should be just 'Item'
        type wrongType = ArrayInnerType<T["items"]>;
        // Then this is OK
        const p: wrongType = new Item();
        return p;
    }
}

const p = new Manager<SuperItemContainer>();
// x: SuperItem, but it's actually an Item
const x = p.manage(new SuperItemContainer());

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.