Re-enable trait forwarders

```
cat sandbox/test1.scala && javap -c -classpath . T C
trait T {
  def m = 42
}

class C extends T

object Test {
  def main(args: Array[String]): Unit = {
    val c = new C()
    assert(c.m == 42)
  }
}
Compiled from "test1.scala"
public interface T {
  public int m();
    Code:
       0: bipush        42
       2: ireturn

  public void $init$();
    Code:
       0: return
}
Compiled from "test1.scala"
public class C implements T {
  public int m();
    Code:
       0: aload_0
       1: invokespecial #14                 // Method T.m:()I
       4: ireturn

  public C();
    Code:
       0: aload_0
       1: invokespecial #20                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: invokespecial #23                 // Method T.$init$:()V
       8: return
}
```

Here's another test that shows private methods working:

```
cat sandbox/test3.scala && qscalac sandbox/test3.scala && javap -private -classpath . -c T C && qscala Test
trait T {
  private def foo = 0
  def bar = foo
}

class C extends T

object Test {
  def main(args: Array[String]): Unit = {
    new C().bar
  }
}
Compiled from "test3.scala"
public interface T {
  private int foo();
    Code:
       0: iconst_0
       1: ireturn

  public int bar();
    Code:
       0: aload_0
       1: invokespecial #15                 // Method foo:()I
       4: ireturn

  public void $init$();
    Code:
       0: return
}
Compiled from "test3.scala"
public class C implements T {
  public int bar();
    Code:
       0: aload_0
       1: invokespecial #14                 // Method T.bar:()I
       4: ireturn

  public C();
    Code:
       0: aload_0
       1: invokespecial #20                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: invokespecial #23                 // Method T.$init$:()V
       8: return
}
```

Author: retronym

src/compiler/scala/tools/nsc/transform/AddInterfaces.scala

@@ -40,7 +40,7 @@ abstract class AddInterfaces extends InfoTransform { self: Erasure =>
   /** Is given trait member symbol a member of the trait's interface
    *  after this transform is performed?
    */
-  private def isInterfaceMember(sym: Symbol) = (
+  def isInterfaceMember(sym: Symbol) = (
     sym.isType || {
       sym.info  // initialize to set lateMETHOD flag if necessary
 

src/compiler/scala/tools/nsc/transform/Mixin.scala

@@ -43,7 +43,9 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
    *     methods in the impl class (because they can have arbitrary initializers)
    */
   private def isImplementedStatically(sym: Symbol) = (
-       sym.owner.isImplClass
+       erasure.isInterfaceMember(sym)
+    && erasure.needsImplMethod(sym)
+    && sym.owner.isTrait
     && sym.isMethod
     && (!sym.isModule || sym.hasFlag(PRIVATE | LIFTED))
     && (!(sym hasFlag (ACCESSOR | SUPERACCESSOR)) || (sym hasFlag LAZY | PARAMACCESSOR) || !(sym hasFlag SYNTHESIZE_IMPL_IN_SUBCLASS))
@@ -266,16 +268,12 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
     )
 
     /* Mix in members of implementation class mixinClass into class clazz */
-    def mixinImplClassMembers(mixinClass: Symbol, mixinInterface: Symbol) {
-      if (!mixinClass.isImplClass) devWarning ("Impl class flag is not set " +
-        ((mixinClass.debugLocationString, mixinInterface.debugLocationString)))
-
+    def mixinTraitForwarders(mixinClass: Symbol) {
       for (member <- mixinClass.info.decls ; if isForwarded(member)) {
-        val imember = member overriddenSymbol mixinInterface
-        imember overridingSymbol clazz match {
+        member overridingSymbol clazz match {
           case NoSymbol =>
-            if (clazz.info.findMember(member.name, 0, lateDEFERRED, stableOnly = false).alternatives contains imember)
-              cloneAndAddMixinMember(mixinInterface, imember).asInstanceOf[TermSymbol] setAlias member
+            if (clazz.info.findMember(member.name, 0, 0L, stableOnly = false).alternatives contains member)
+              cloneAndAddMixinMember(mixinClass, member).asInstanceOf[TermSymbol] setAlias member
           case _        =>
         }
       }
@@ -358,12 +356,12 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
     // first complete the superclass with mixed in members
     addMixedinMembers(clazz.superClass, unit)
 
-    for (mc <- clazz.mixinClasses ; if mc hasFlag lateINTERFACE) {
+    for (mc <- clazz.mixinClasses ; if mc.isTrait) {
       // @SEAN: adding trait tracking so we don't have to recompile transitive closures
       unit.depends += mc
       addLateInterfaceMembers(mc)
       mixinTraitMembers(mc)
-      mixinImplClassMembers(implClass(mc), mc)
+      mixinTraitForwarders(mc)
     }
   }
 
@@ -372,53 +370,7 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
    *   - Implementation classes become modules that inherit nothing
    *     and that define all.
    */
-  override def transformInfo(sym: Symbol, tp: Type): Type = tp match {
-    case ClassInfoType(parents, decls, clazz) =>
-      var parents1 = parents
-      var decls1 = decls
-      if (!clazz.isPackageClass) {
-        exitingMixin(clazz.owner.info)
-        if (clazz.isImplClass) {
-          clazz setFlag lateMODULE
-          var sourceModule = clazz.owner.info.decls.lookup(sym.name.toTermName)
-          if (sourceModule == NoSymbol) {
-            sourceModule = (
-              clazz.owner.newModuleSymbol(sym.name.toTermName, sym.pos, MODULE)
-                setModuleClass sym.asInstanceOf[ClassSymbol]
-            )
-            clazz.owner.info.decls enter sourceModule
-          }
-          else {
-            sourceModule setPos sym.pos
-            if (sourceModule.flags != MODULE) {
-              log(s"!!! Directly setting sourceModule flags for $sourceModule from ${sourceModule.flagString} to MODULE")
-              sourceModule.flags = MODULE
-            }
-          }
-          sourceModule setInfo sym.tpe
-          // Companion module isn't visible for anonymous class at this point anyway
-          assert(clazz.sourceModule != NoSymbol || clazz.isAnonymousClass,  s"$clazz has no sourceModule: $sym ${sym.tpe}")
-          parents1 = List()
-          decls1 = newScopeWith(decls.toList filter isImplementedStatically: _*)
-        } else if (!parents.isEmpty) {
-          parents1 = parents.head :: (parents.tail map toInterface)
-        }
-      }
-      //decls1 = enteringPhase(phase.next)(newScopeWith(decls1.toList: _*))//debug
-      if ((parents1 eq parents) && (decls1 eq decls)) tp
-      else ClassInfoType(parents1, decls1, clazz)
-
-    case MethodType(params, restp) =>
-      toInterfaceMap(
-        if (isImplementedStatically(sym)) {
-          val ownerParam = sym.newSyntheticValueParam(toInterface(sym.owner.typeOfThis))
-          MethodType(ownerParam :: params, restp)
-        } else
-          tp)
-
-    case _ =>
-      tp
-  }
+  override def transformInfo(sym: Symbol, tp: Type): Type = tp
 
   /** Return a map of single-use fields to the lazy value that uses them during initialization.
    *  Each field has to be private and defined in the enclosing class, and there must
@@ -1108,7 +1060,7 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
               val rhs =
                 enteringPickler(sym.info) match {
                   case ct: ConstantType => gen.mkAttributedQualifier(ct) // don't call forwarder if it's just going to return a literal (no need for this in the new trait encoding)
-                  case _ => Apply(staticRef(sym.alias), gen.mkAttributedThis(clazz) :: sym.paramss.head.map(Ident))
+                  case _ => Apply(Select(Super(clazz, tpnme.EMPTY), sym.alias), sym.paramss.head.map(Ident))
                 }
               addDefDef(sym, rhs)
             }