Using ClassTag with Variance - scala

Question

I have a transfomer which is just a scala function, but needs ClassTag for interop with legacy code:
trait Transformer[F, T] extends (F => T) {
implicit def cF: ClassTag[F]
implicit def cT: ClassTag[T]
/**
* Append another transformer to this transformer
*/
def >=>[U: ClassTag](f: Transformer[T, U]): Transformer[F, U] = Transformer(this andThen f)
}
/** Helper to view a function as a transformer */
object Transformer {
def apply[F: ClassTag, T: ClassTag](f: F => T): Transformer[F, T] = new AbstractTransformer[F, T] {
override def apply(x: F): T = f(x)
}
}
abstract class AbstractTransformer[F, T](implicit override val cF: ClassTag[F], override val cT: ClassTag[T]) extends Transformer[F, T]
However, I would like to have the same variance in T and F as scala
Function1 which is
trait Transformer[-F, +T] extends (F => T)
However this results in
contravariant type F occurs in invariant position in type =>
scala.reflect.ClassTag[F] of method cF Transformer.scala
covariant type T occurs in invariant position in type =>
scala.reflect.ClassTag[T] of method cT Transformer.scala
Can I solve this problem somehow?

Answer 1

import scala.annotation.unchecked.uncheckedVariance
...
implicit def cF: ClassTag[F #uncheckedVariance]
implicit def cT: ClassTag[T #uncheckedVariance]
However, you need to be careful using this: there is a reason for the error! For example, consider
val t: Transformer[String, Object] = ...
val t1: Transformer[Object, String] = t // legal by variance
val c1 = t1.cF // the compiler thinks it's a ClassTag[Object], but really it's the ClassTag for String
val c2 = t1.cT // vice versa