Question: Why can't type parameters be used inside type parameters?

[ryasmi]

Hey all, hoping to be educated by someone. I think I need to reuse type parameters inside type parameters (see example code below, specifically the reuse of Y), but this is firstly not allowed by TypeScript (and perhaps other languages too), and secondly it seems "odd". So I'm wondering, why can't type parameters be used inside type parameters? Is there a solution to this I'm missing?

class A {
  // ...
}

class B<Z extends A> {
  public a: Z;
  // ...
}

class C<Y extends A, X extends B<Y>> {
  public a: Y;
  public b: X;
  // ...
}

[ryasmi]

Thanks for your response @saschanaz. That's the solution I'm using to overcome this at the moment, I'm just wondering if there is perhaps a better one, since to extend that class using a subclass of B I must override the parameter (see class E below).

class D<Z> extends B<Z> {
  // ...
}

class E<Y> extends C<Y> {
  public b: D<Y>;
  // ...
}

[ryasmi]

Has @saschanaz deleted their comment?

I think that is to prevent circular reference. Will this solve your problem?

class C<Y extends A> {
  public a: Y;
  public b: B<Y>
}

[saschanaz]

@ryansmith94 Ah, I thought my answer was not 100% right so I deleted it, sorry.

[ryasmi]

Ah no worries, I wouldn't worry about posting something you're unsure about as long as you say that you're not sure 😄

[DanielRosenwasser]

Try installing the nightly of TypeScript (npm install -g typescript@next) where we should now support type parameters referencing other type parameters (see #5949 and https://github.com/Microsoft/TypeScript/wiki/Roadmap#18).

[ryasmi]

Awesome! Thanks @DanielRosenwasser 👍