New trait encoding

0.test.scala

trait T {
  println("T")
  def m1 = m2
  private def m2 = "m2"
}
trait U extends T {
  println("T")
  override def m1 = super[T].m1
}
class C extends U {
  println("C")
  def test = m1
}

1.old.log

public interface T {
  public abstract java.lang.String m1();
}
public abstract class T$class {
  public static java.lang.String m1(T);
    Code:
       0: aload_0
       1: invokestatic                      // Method m2:(LT;)Ljava/lang/String;
       4: areturn

  private static java.lang.String m2(T);
    Code:
       0: ldc                               // String m2
       2: areturn

  public static void $init$(T);
    Code:
       0: getstatic                         // Field scala/Predef$.MODULE$:Lscala/Predef$;
       3: ldc                               // String T
       5: invokevirtual                     // Method scala/Predef$.println:(Ljava/lang/Object;)V
       8: return
}
public interface U extends T {
  public abstract java.lang.String m1();
}
public abstract class U$class {
  public static java.lang.String m1(U);
    Code:
       0: aload_0
       1: invokestatic                      // Method T$class.m1:(LT;)Ljava/lang/String;
       4: areturn

  public static void $init$(U);
    Code:
       0: getstatic                         // Field scala/Predef$.MODULE$:Lscala/Predef$;
       3: ldc                               // String T
       5: invokevirtual                     // Method scala/Predef$.println:(Ljava/lang/Object;)V
       8: return
}
public class C implements U {
  public java.lang.String m1();
    Code:
       0: aload_0
       1: invokestatic                      // Method U$class.m1:(LU;)Ljava/lang/String;
       4: areturn

  public java.lang.String test();
    Code:
       0: aload_0
       1: invokevirtual                     // Method m1:()Ljava/lang/String;
       4: areturn

  public C();
    Code:
       0: aload_0
       1: invokespecial                     // Method java/lang/Object."<init>":()V
       4: aload_0
       5: invokestatic                      // Method T$class.$init$:(LT;)V
       8: aload_0
       9: invokestatic                      // Method U$class.$init$:(LU;)V
      12: getstatic                         // Field scala/Predef$.MODULE$:Lscala/Predef$;
      15: ldc                               // String C
      17: invokevirtual                     // Method scala/Predef$.println:(Ljava/lang/Object;)V
      20: return
}

2.new.log

public interface T {
  public java.lang.String m1();
    Code:
       0: aload_0
       1: invokespecial                     // Method m2:()Ljava/lang/String;
       4: areturn

  private java.lang.String m2();
    Code:
       0: ldc                               // String m2
       2: areturn

  public void $init$();
    Code:
       0: getstatic                         // Field scala/Predef$.MODULE$:Lscala/Predef$;
       3: ldc                               // String T
       5: invokevirtual                     // Method scala/Predef$.println:(Ljava/lang/Object;)V
       8: return
}
public interface U extends T {
  public java.lang.String m1();
    Code:
       0: aload_0
       1: invokespecial                     // Method T.m1:()Ljava/lang/String;
       4: areturn

  public void $init$();
    Code:
       0: getstatic                         // Field scala/Predef$.MODULE$:Lscala/Predef$;
       3: ldc                               // String T
       5: invokevirtual                     // Method scala/Predef$.println:(Ljava/lang/Object;)V
       8: return
}
public class C implements U,T {
  public java.lang.String m1();
    Code:
       0: aload_0
       1: invokespecial                     // Method U.m1:()Ljava/lang/String;
       4: areturn

  public java.lang.String test();
    Code:
       0: aload_0
       1: invokevirtual                     // Method m1:()Ljava/lang/String;
       4: areturn

  public C();
    Code:
       0: aload_0
       1: invokespecial                     // Method java/lang/Object."<init>":()V
       4: aload_0
       5: invokespecial                     // Method T.$init$:()V
       8: aload_0
       9: invokespecial                     // Method U.$init$:()V
      12: getstatic                         // Field scala/Predef$.MODULE$:Lscala/Predef$;
      15: ldc                               // String C
      17: invokevirtual                     // Method scala/Predef$.println:(Ljava/lang/Object;)V
      20: return
}

3.diff

--- 1.old.log	2016-03-18 10:52:15.000000000 +1000
+++ 2.new.log	2016-03-18 10:52:18.000000000 +1000
@@ -1,65 +1,59 @@
 public interface T {
-  public abstract java.lang.String m1();
-}
-public abstract class T$class {
-  public static java.lang.String m1(T);
+  public java.lang.String m1();
     Code:
        0: aload_0
-       1: invokestatic                      // Method m2:(LT;)Ljava/lang/String;
+       1: invokespecial                     // Method m2:()Ljava/lang/String;
        4: areturn
 
-  private static java.lang.String m2(T);
+  private java.lang.String m2();
     Code:
        0: ldc                               // String m2
        2: areturn
 
-  public static void $init$(T);
+  public void $init$();
     Code:
        0: getstatic                         // Field scala/Predef$.MODULE$:Lscala/Predef$;
        3: ldc                               // String T
        5: invokevirtual                     // Method scala/Predef$.println:(Ljava/lang/Object;)V
        8: return
 }
 public interface U extends T {
-  public abstract java.lang.String m1();
-}
-public abstract class U$class {
-  public static java.lang.String m1(U);
+  public java.lang.String m1();
     Code:
        0: aload_0
-       1: invokestatic                      // Method T$class.m1:(LT;)Ljava/lang/String;
+       1: invokespecial                     // Method T.m1:()Ljava/lang/String;
        4: areturn
 
-  public static void $init$(U);
+  public void $init$();
     Code:
        0: getstatic                         // Field scala/Predef$.MODULE$:Lscala/Predef$;
        3: ldc                               // String T
        5: invokevirtual                     // Method scala/Predef$.println:(Ljava/lang/Object;)V
        8: return
 }
-public class C implements U {
+public class C implements U,T {
   public java.lang.String m1();
     Code:
        0: aload_0
-       1: invokestatic                      // Method U$class.m1:(LU;)Ljava/lang/String;
+       1: invokespecial                     // Method U.m1:()Ljava/lang/String;
        4: areturn
 
   public java.lang.String test();
     Code:
        0: aload_0
        1: invokevirtual                     // Method m1:()Ljava/lang/String;
        4: areturn
 
   public C();
     Code:
        0: aload_0
        1: invokespecial                     // Method java/lang/Object."<init>":()V
        4: aload_0
-       5: invokestatic                      // Method T$class.$init$:(LT;)V
+       5: invokespecial                     // Method T.$init$:()V
        8: aload_0
-       9: invokestatic                      // Method U$class.$init$:(LU;)V
+       9: invokespecial                     // Method U.$init$:()V
       12: getstatic                         // Field scala/Predef$.MODULE$:Lscala/Predef$;
       15: ldc                               // String C
       17: invokevirtual                     // Method scala/Predef$.println:(Ljava/lang/Object;)V
       20: return
 }

Generated with:

scalac 0.test.scala && javap -c -private T 'T$class' U 'U$class' C 2>&1 | grep -v "Compiled from" | perl -pe 's/#\d+/   /' > 1.old.log && qscalac 0.test.scala && javap -c -private T U C 2>&1 | grep -v "Compiled from" | perl -pe 's/#\d+/   /' > 2.new.log && diff -U1000 1.old.log 2.new.log > 3.diff