Null should be subtype of non-value classes

[hearnadam]

/** `Null` is - together with [[scala.Nothing]] - at the bottom of the Scala type hierarchy.
  *
  * `Null` is the type of the `null` literal. It is a subtype of every type
  * except those of value classes. Value classes are subclasses of [[AnyVal]], which includes
  * primitive types such as [[Int]], [[Boolean]], and user-defined value classes.
  *
  * Since `Null` is not a subtype of value types, `null` is not a member of any such type.
  * For instance, it is not possible to assign `null` to a variable of type [[scala.Int]].
  */
sealed trait Null

import izumi.reflect.Tag

Tag[Null] <:< Tag[String] // returns false

[pshirshov]

Do you really have a practical usecase for this? I've intentionally made our rules simpler than they are in the compiler in order to save a couple pennies of the performance.

If you really need this, new special cases should be introduced around this place: https://github.com/zio/izumi-reflect/blob/develop/izumi-reflect/izumi-reflect/src/main/scala/izumi/reflect/macrortti/LightTypeTagInheritance.scala#L74

[hearnadam]

I don't have a direct usecase, I was actually testing my own Tag encoding against izumi and hit this. I suspect it would be only useful in unions, if that enables you any optimization:

[pshirshov]

my own Tag encoding against izumi

May I have a look please? I would be happy to add it into our list of relevant projects.

[hearnadam]

I haven't worked on it in some time, but the idea builds on the existing kyo-tag system, but encoded unions and intersections in a single String: https://github.com/getkyo/kyo/pull/948/files

This has many benefits on the JVM, but does not lead to a simple implementation. There are some core bugs I need to solve.

[pshirshov]

Interesting. Is it a scala3-only project?