Fails to compile with: /home/mononofu/tmp/minimal/minimal.scala:7: erroneous or inaccessible type [error] def plus(x: Measure[T],y: Measure[T]): Measure[T] = x + y

build.sbt

scalaVersion in ThisBuild := "2.10.0"

scalacOptions in ThisBuild += "-language:experimental.macros"

libraryDependencies in ThisBuild ++= Seq(
  "org.scala-lang" % "scala-reflect" % "2.10.0"
)

minimal.scala

import language.experimental.macros
import scala.reflect.macros.Context
import collection.mutable.ListBuffer
import scala.reflect.runtime.universe.{WeakTypeTag, TypeRef, TypeTag}

class MyNumeric[T] extends Numeric[Measure[T]] {
  def plus(x: Measure[T],y: Measure[T]): Measure[T] = x + y
}

class Measure[T](val n: Int) extends AnyVal {
  def +[U](that: Measure[U])(implicit tag: WeakTypeTag[T], tag2: WeakTypeTag[U]) =
    macro MeasureImpl.addition_impl[T, U]
}

object MeasureImpl {
  def precompute(c: Context)(a: c.Tree, b: c.Tree) = {
    import c.universe._
    val evals = ListBuffer[ValDef]()

    def _precompute(value: Tree, tpe: Type): Ident = {
      val freshName = newTermName(c.fresh("eval$"))
      evals += ValDef(Modifiers(), freshName, TypeTree(tpe), value)
      Ident(freshName)
    }

    val aID = _precompute(a, typeOf[Measure[_]])
    val bID = _precompute(b, typeOf[Measure[_]])
    (evals, aID, bID)
  }

  def addition_impl[T: c.WeakTypeTag, U: c.WeakTypeTag]
    (c: Context)
    (that: c.Expr[Measure[U]])
    (tag: c.Expr[WeakTypeTag[T]], tag2: c.Expr[WeakTypeTag[U]]): c.Expr[Any] = {

    import c.universe._

    val resultType = c.universe.AppliedTypeTree(
          c.universe.Ident(c.universe.newTypeName("SUnit")),
          List(c.universe.Ident(c.universe.newTypeName("Meter")),
               c.universe.Ident(c.universe.newTypeName("Pos1"))))

    val (evals, aID, bID) = precompute(c)(that.tree, c.prefix.tree)

    val stats = Apply(Select(New(AppliedTypeTree(
      Ident(newTypeName("Measure")),
      List( resultType )
      )), nme.CONSTRUCTOR),
      List(Apply(Select(
        Select(
          Ident(newTermName(aID.toString)),
          newTermName("n")),
        newTermName("$plus")),
          List(
            Select(
              Ident(newTermName(bID.toString)),
              newTermName("n"))))))


    c.Expr(Block(evals.toList, stats))
  }
}