False-positive subtype checking for Scala 3 opaque types

[kyri-petrou]

For some reason, when 2 Scala 3 opaque types have the same underlying type, the subtype check <:< seems to falsely return true. AFAICT, this seems to be what's causing zio/zio#8882 (albeit there might be more to it, but first things first 😅)

Repro:

import izumi.reflect.Tag

type UserName = UserName.T
object UserName {
  opaque type T = String
}

type UserId = UserId.T
object UserId {
  opaque type T = String
}

val idTag   = Tag[UserId]
val nameTag = Tag[UserName]

@main def m = {
  assert(!(idTag =:= nameTag), "idTag =:= nameTag") // passes
  assert(!(idTag <:< nameTag), "idTag <:< nameTag") // fails
}

[neko-kai]

Addressed in #468