Extend generic static bug

[mushishi78]

TypeScript Version: 2.4.0

Code

function base<T>() {
    class Base {
        static prop: T;
    }
    return Base;
}

class Gen<T> extends base<T>() {}
class Spec extends Gen<string> {}

<string>Spec.prop;

Expected behavior: Either the static property prop is of type string or I'm given an error to prevent this happening earlier with clear error message.

Actual behavior: It claims that prop is of type T.

[ahejlsberg]

The issue here is that type parameters introduced by a class declaration should not be in scope within the expression part of the base class heritage clause. However, when a type argument list follows the expression part, they should be in scope there. So, this should be an error

class Gen<T> extends base<T>() {}  // Error, T not in scope in base class expression

[markboyall]

Then... how would you implement the Gen class above? Or would it need to be wrapped as well?

I'm not even sure what the lifetime of the static inside the base class is, whether it's one per call of base, or just one.

[ahejlsberg]

@markboyall Yes, you'd need to wrap the derived class in a function as well such that you can pass a type parameter that differs per invocation. For example:

function base<T>() {
    class Base {
        static prop: T;
    }
    return Base;
}

function derived<T>() {
    class Derived extends base<T>() {
        static anotherProp: T;
    }
    return Derived;
}

class Spec extends derived<string>() {}

Spec.prop;  // string
Spec.anotherProp;  // string

The lifetime of the statics above is one per invocation of their containing function. Effectively, a class declaration manufactures a class constructor function each time it is executed (which is only once for a global class, but can be many times for a class nested in a function).