Specialized case class does not compile

[Atry]

Welcome to Scala 2.12.5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_162).
Type in expressions for evaluation. Or try :help.

scala> case class C[A, @specialized B](handler: B => A)
<console>:11: error: type mismatch;
 found   : Byte => A(in class C$mcB$sp)
 required: Byte => A(in method copy$default$1$mcB$sp)
       case class C[A, @specialized B](handler: B => A)
                                       ^