Fix to trait setter generation

The logic to add trait setters to Scala 2 traits was wrong. This led
to AbstractMethodErrors in the formerly commented out part of scala-trait.scala.

Author: odersky

src/dotty/tools/dotc/transform/AugmentScala2Traits.scala

@@ -69,16 +69,13 @@ class AugmentScala2Traits extends MiniPhaseTransform with IdentityDenotTransform
           info = fullyParameterizedType(mold.info, mixin))
       }
 
-      def traitSetter(getter: TermSymbol) = {
-        val separator = if (getter.is(Private)) nme.EXPAND_SEPARATOR else nme.TRAIT_SETTER_SEPARATOR
-        val expandedGetterName =
-          if (getter.is(ExpandedName)) getter.name
-          else getter.name.expandedName(getter.owner, separator)
+      def traitSetter(getter: TermSymbol) =
         getter.copy(
-          name = expandedGetterName.setterName,
+          name = getter.ensureNotPrivate.name
+                  .expandedName(getter.owner, nme.TRAIT_SETTER_SEPARATOR)
+                  .asTermName.setterName,
           flags = Method | Accessor | ExpandedName,
           info = MethodType(getter.info.resultType :: Nil, defn.UnitType))
-      }
 
       for (sym <- mixin.info.decls) {
         if (needsForwarder(sym) || sym.isConstructor || sym.isGetter && sym.is(Lazy))

tests/pos/scala2traits/scala-trait.scala

@@ -9,7 +9,7 @@ trait T {
 trait S2T {
   var x: Int = 0
   lazy val y: Int = 1
-//  val z: Int = 2
+  val z: Int = 2
   val a: Int
   var b: Int
 
@@ -19,13 +19,13 @@ trait S2T {
 trait S2Tprivate {
   private var x: Int = 0
   private lazy val y: Int = 1
-//  private val z: Int = 2  // @darkdimius uncomment once lazy vals can be inherited.
+  private val z: Int = 2
 
   private def f(x: Int): Int = x + y
   def xx = x
   def xx_=(x: Int) = this.x = x
   def yy = y
-//  def zz = z
+  def zz = z
   def ff(x: Int) = f(x)
 }