How can I apply a macro annotation to a case class with a context bound?

问题

When I try to add a macro annotation to my case class:

@macid case class CC[A: T](val x: A)

I get the error:

private[this] not allowed for case class parameters

@macid is just the identity function, defined as a whitebox StaticAnnotation:

import scala.language.experimental.macros
import scala.reflect.macros.whitebox.Context
import scala.annotation.StaticAnnotation
class macid extends StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro macidMacro.impl
}
object macidMacro {
  def impl(c: Context)(annottees: c.Expr[Any]*): c.Expr[Any] = {
    new Macros[c.type](c).macidMacroImpl(annottees.toList)
  }
}
class Macros[C <: Context](val c: C) {
  import c.universe._
  def macidMacroImpl(annottees: List[c.Expr[Any]]): c.Expr[Any] =
    annottees(0)
}

The unannotated code works:

case class CC[A: T](val x: A)

It works if I remove the context bound:

@macid case class CC[A](val x: A)

What's happening is the context bound is desugared into a private parameter. The following desugared code gets the same error:

@macid case class CC[A](val x: A)(implicit aIsT: T[A])

To get working code I make the implicit parameter public with val:

@macid case class CC[A](val x: A)(implicit val aIsT: T[A])

So my questions are: What's the right way for a macro annotation to support context bounds? Why does the compiler perform a no-private-parameters-of-case-classes check for code that's generated by a macro annotation, but doesn't perform the check for ordinary code?

Scala versions 2.11.7 and 2.12.0-M3 both report the error. All the above code examples compile and run as expected in 2.11.3.

回答1:

Seems like a bug. Here is the tree as seen by the macro:

case class CC[A] extends scala.Product with scala.Serializable {
  <caseaccessor> <paramaccessor> val x: A = _;
  implicit <synthetic> <caseaccessor> <paramaccessor> private[this] val evidence$1: T[A] = _;
  def <init>(x: A)(implicit evidence$1: T[A]) = {
    super.<init>();
    ()
  }
}

And through the runtime reflection API:

case class CC[A] extends Product with Serializable {
  <caseaccessor> <paramaccessor> val x: A = _;
  implicit <synthetic> <paramaccessor> private[this] val evidence$1: $read.T[A] = _;
  def <init>(x: A)(implicit evidence$1: $read.T[A]) = {
    super.<init>();
    ()
  }
};

The former has an extra <caseaccessor> flag on evidence$1 when it should not. It seems as though all implicit parameters to case classes are mistakenly given this flag.

来源：https://stackoverflow.com/questions/35468913/how-can-i-apply-a-macro-annotation-to-a-case-class-with-a-context-bound

标签

scala

scala-macros

case-class

context-bound