HTMLOrSVGElement does not extend Element

[MarsiBarsi]

TypeScript Version: any

Expected behavior:
HTMLOrSVGElement extends Element

Actual behavior:
it does not

Playground Link: https://www.typescriptlang.org/play?ts=4.2.0-dev.20201112#code/DYUwLgBAhgXBASAVAsgGQPICcDKA1A4gKKgC2IAdmANwBQokARnMSGZbTQxALzRVA

[RyanCavanaugh]

This is correct; Element lacks some things that are in both HTMLElement and SVGElement

[MarsiBarsi]

Thank you for answer, @RyanCavanaugh!
Sure. My proposal is to extend HTMLOrSVGElement with Element because both HTMLElement and SVGElement are actually Element

[RyanCavanaugh]

The interface describes the common properties between HTMLElement and SVGElement. It's still not safe to assign an arbitrary Element to that

If you have DogOrCat, you still can't assign a Mammal to it, because there are Mammals that aren't Dogs or Cats even though both of those are Mammals. This would only be valid if you could prove that there are no other Mammals except Dog or Cat, but this isn't the case.

[waterplea]

@RyanCavanaugh I believe you are confused because playground link is incorrect example. With your example of DogOrCat interface and Mammal — DogOrCat IS a Mammal therefore DogOrCat should extend Mammal. Right now, say, if Mammal interface has milk property, DogOrCat interface doesn't have it, although it should.

[RyanCavanaugh]

I don't disagree, but the example provided attempts to assign an Element to an HTMLOrSVGElement, which still shouldn't be allowed.

[MarsiBarsi]

Yes, I am very sorry :) I have meant to show this: https://www.typescriptlang.org/play?ts=4.2.0-dev.20201112#code/DYUwLgBAhgXBASAVAsgGQPICcDKA1A4gKKgC2IAdmANwBQokARnMSGZbTQxALzRVA

[ExE-Boss]

The current behaviour is correct, because HTMLOrSVGElement is an interface mixin:

interface mixin HTMLOrSVGElement {
  [SameObject] readonly attribute DOMStringMap dataset;
  attribute DOMString nonce; // intentionally no [CEReactions]

  [CEReactions] attribute boolean autofocus;
  [CEReactions] attribute long tabIndex;
  undefined focus(optional FocusOptions options = {});
  undefined blur();
};