Workaround for Scala macro annotation bug

Question

I have a macro annotation that I use to inject implicit type class to a companion method.

@MyMacro case class MyClass[T](a: String, b: Int, t: T)

Most of the time it work as expected, but it breaks when I use type constraint notation:

@MyMacro case class MyClass[T: TypeClass](a: String, b: Int, t: T)
// private[this] not allowed for case class parameters

This error was described on SO and reported as a bug.

Thing is: macros (v1) are no longer maintained, so I cannot expect that this will be fixed.

So what I wanted to know is: can I fix this myself within a macro? Is this change done to AST in a way that I could somehow undo it? I would like to try repairing it within a macro instead of forcing all users to rewrite their code to ...(implicit tc: TypeClass[T]).

余杰水余杰水 · Accepted Answer · 2018-11-07T03:07:48

class AnnotationType() extends scala.annotation.StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro AnnotationTypeImpl.impl
}

class AnnotationTypeImpl(val c: blackbox.Context) {

  import c.universe._

  def impl(annottees: Tree*): Tree = {

    val tree = annottees.head.asInstanceOf[ClassDef]

    val newTree = tree match {
      case ClassDef(mods, name, tparams, impl@Template(parents, self, body)) =>
        val newBody = body.map {
          case ValDef(mods, name, tpt, rhs) =>
            // look here
            // the flag of `private[this]` is Flag.PRIVATE | Flag.LOCAL
            // the flag of `private` is Flag.PRIVATE
            // drop Flag.LOCAL in Modifiers.flags ,  it will change `private[this]` to `private`
            val newMods =
              if(mods.hasFlag(Flag.IMPLICIT))
                mods.asInstanceOf[scala.reflect.internal.Trees#Modifiers].&~(Flag.LOCAL.asInstanceOf[Long]).&~(Flag.CASEACCESSOR.asInstanceOf[Long]).asInstanceOf[Modifiers]
              else
                mods
            ValDef(newMods, name, tpt, rhs)

          case e => e
        }
        ClassDef(mods, name, tparams, Template(parents, self, newBody))
    }
    println(show(tree))
    println(show(newTree))

    q"..${newTree +: annottees.tail}"
  }
}

// test

@AnnotationType()
case class AnnotationTypeTest[T: Option](a: T){
  def option = implicitly[Option[T]]
}

object AnnotationTypeTest {
  def main(args: Array[String]): Unit = {
    implicit val x = Option(1)

    println(AnnotationTypeTest(100))
    println(AnnotationTypeTest(100).option)
    println(AnnotationTypeTest(100).copy(a =2222))
    println(AnnotationTypeTest(100).copy(a =2222)(Some(999)).option)
  }

}

Workaround for Scala macro annotation bug

1 Answers