java.lang.constant.Constable should be treated as a transparent trait to help Scala.js

[sjrd]

See the discussion at typelevel/shapeless-3#1 (comment)

On JDK 12+, java.lang.constant.Constable can come up as an inferred LUB of primitive classes like String and Integer. This is problematic on Scala.js (and in the future in Scala Native) since the API of Constable cannot be implemented, causing linking errors.

In Scala 2, there is not much we can do about it except using explicit type ascriptions to avoid it being inferred. In Scala 3 however, this would easily be solved by treating Constable as a transparent trait, like is done (IIRC) for java.lang.Comparable.

[joroKr21]

Note that the problem in shapeless is caused by the compiler generated productElement of

case class ValueTypeable[T, B](cB: Class[B], describe: String)