is this a Generic bug?

[Paul-Lo]

consider the following class.

class GenericPractice<T>{
    private entity: T;
    constructor(entity: T) {
        this.entity = entity;
    }

    public add(item: T): string {
        return item.toString();
    }

}
class CouponInfo {
    public toString(): string {
        return "couponInfo";
    }
}
class Snake{
}

 var genericPractice = new GenericPractice<CouponInfo>(new CouponInfo());
genericPractice.add(new Snake("Sammy the Python")));

theoretically, the above last line:

genericPractice.add(new Snake("Sammy the Python")));

should have compile time error since the generic of class GenericPractice should only allow CouponInfo in method add, not Snake.

However, the compile still passes which violates the fundamental concept of generic.
is this a bug?

[DanielRosenwasser]

Note: I assume that you didn't mean to supply an argument to the Snake constructor.

This has to do with TypeScript having a predominantly structural type system. For that reason, the following works:

class CouponInfo {
    public toString(): string {
        return "couponInfo";
    }
}

class Snake {
}

// No problem with either of these.
var x: CouponInfo = new Snake();
var y: Snake = new CouponInfo();

Since instances of Snake have the same apparent members as those of CouponInfo, Snakes are assignable to CouponInfos.

Thus when add expects a CouponInfo, you can give it a Snake.

If you want to avoid this, you can add a private "tag" member in each class. Privates are not compared in a structural manner, regardless of their name and type.

class CouponInfo {
    // Introduced a tag here.
    private couponTag: {};

    public toString(): string {
        return "couponInfo";
    }
}

class Snake {
    // Introduced a tag here.
    private snakeTag: {};
}

var genericPractice = new GenericPractice(new CouponInfo());

// Now all of these cause errors.
genericPractice.add(new Snake());
var x: CouponInfo = new Snake();
var y: Snake = new CouponInfo();

Hope that answers your question!

[Paul-Lo]

Thanks for the informative and comprehensive answer!