How to check I'm inside a @specialized function or class at runtime in scala?

Question

Let's say I have a specialized class and an associated companion object:

trait Slice[@specialized +T] {
    ...

    override def equals(that :Any) = that match {
        case s :Slice[_] => ???
        case _ => false
    }
}

object Slice {
    def newInstance[@specialized T] = ???
}

Is there any way to check

1. Inside a method of Slice if this instance is a specialized subclass,
2. Inside a method of Slice if another instance is a specialized subclass for the same primitive,
3. Inside a specialized method on a companion object if I'm running an erased or specialized variant

without resorting to ClassTags or passing Class[_] manually? It seems like that information should be available, but the only way I can think of involves checking names of the classes.

Use case 2) is particularly important, as I could resort to faster algorithms if I knew I'm comparing apples with apples. It probably could be accomplished by reflection, but it would be quite tricky when you take into account that we have to handle non-synthetic subclasses of Slice, too; if we have also

trait ArraySlice[@specialized T] extends Slice[T] { ... }

that should be considered 'compatible' with Slice[T] instances as long as they are both specialized (or both erased)?

Answer 1

Ok, I figured out a cleaner way:

final val AllButUnit = new Specializable.Group((Byte, Short, Int, Long, Char, Float, Double, Boolean, AnyRef))

def specializationFor[@specialized(AllButUnit) E] :ResolvedSpecialization[E] =
    Specializations(new SpecializedKey[E]).asInstanceOf[ResolvedSpecialization[E]]


private val Specializations = Seq(
    resolve[Byte],
    resolve[Short],
    resolve[Int],
    resolve[Long],
    resolve[Char],
    resolve[Float],
    resolve[Double],
    resolve[Boolean],
    resolve[Unit],
    resolve[AnyRef]
).map(
    spec => spec.key -> spec :(SpecializedKey[_], ResolvedSpecialization[_])
).toMap.withDefaultValue(resolve[AnyRef])

private def resolve[@specialized(AllButUnit) E :ClassTag] :ResolvedSpecialization[E] =
    new ResolvedSpecialization[E](new SpecializedKey[E], new Array[E](0))


class ResolvedSpecialization[@specialized(AllButUnit) E] private[SpecializedCompanion]
        (val array :Array[E], val elementType :Class[E], val classTag :ClassTag[E], private[SpecializedCompanion] val key :SpecializedKey[E])
{
    private[SpecializedCompanion] def this(key :SpecializedKey[E], array :Array[E]) =
        this(array, array.getClass.getComponentType.asInstanceOf[Class[E]], ClassTag(array.getClass.getComponentType.asInstanceOf[Class[E]]), key)

    override def toString = s"@specialized($elementType)"

    override def equals(that :Any) = that match {
        case r :ResolvedSpecialization[_] => r.elementType==elementType
        case _ => false
    }

    override def hashCode = elementType.hashCode
}

private class SpecializedKey[@specialized(AllButUnit) E] {
    override def equals(that :Any) = that.getClass==getClass
    override def hashCode = getClass.hashCode

    def className = getClass.getName
    override def toString = className.substring(className.indexOf("$")+1)
}

Now specializationFor[E].elementType returns class corresponding to specialization parameter of E.