Opaque type alias evidence

[cchantep]

It would nice that opaque type aliases were given evidence at typelevel. e.g.

def funThatExceptOpaqueTypeAlias[T: OpaqueAlias] = println("ok")

opaque type Foo = Double

funThatExceptOpaqueTypeAlias[Foo] // ok

funThatExceptOpaqueTypeAlias[Double] // do not compile

Something as below can be designed.

sealed trait OpaqueAlias[T] {}

object OpaqueAlias:
  import scala.quoted.{ Expr, Quotes, Type }

  inline given materialized[T]: OpaqueAlias[T] = ${ impl[T] }

  private def impl[T: Type](using q: Quotes): Expr[OpaqueAlias[T]] = {
    import q.reflect.*

    TypeRepr.of[T] match {
      case ref: TypeRef if ref.isOpaqueAlias =>
        ref.asType match {
          case tpe @ '[t] => '{ new OpaqueAlias[T] {} }
        }

      case tpr =>
        report.errorAndAbort(s"${tpr.show} is not an opaque alias")
    }
  }

end OpaqueAlias

[bishabosha]

What do you imagine using this evidence for?

[cchantep]

To apply some function only to such type (for example macro that could then resolve the super translucent type behind).

[ivan-klass]

Might be useful in type-class derivation to break the loop