implicit value class vs. implicit def + value class

A.scala

case class A(i: Int)

object A {
  implicit class ASyntax(val self: A) extends AnyVal {
    def twice: Int = self.i * 2
  }

  implicit def toASyntax2(a: A): ASyntax2 =
    new ASyntax2(a)
}

A2.scala

class ASyntax2(val self: A) extends AnyVal {
  def once: Int = self.i
}

B.scala

class B {
  val a = A(1)
  a.once
  a.twice
}

javapB.txt

Classfile /home/frank/syn/B.class
  Last modified Mar 31, 2015; size 942 bytes
  MD5 checksum 7d0367f3829360308b4da60e28045012
  Compiled from "B.scala"
public class B
  SourceFile: "B.scala"
  InnerClasses:
       public static #57= #47 of #21; //ASyntax$=class A$ASyntax$ of class A
  RuntimeVisibleAnnotations:
    0: #6(#7=s#8)
    ScalaSig: length = 0x3
     05 00 00 
  minor version: 0
  major version: 50
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Utf8               B
   #2 = Class              #1             //  B
   #3 = Utf8               java/lang/Object
   #4 = Class              #3             //  java/lang/Object
   #5 = Utf8               B.scala
   #6 = Utf8               Lscala/reflect/ScalaSignature;
   #7 = Utf8               bytes
   #8 = Utf8               
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   #9 = Utf8               a
  #10 = Utf8               LA;
  #11 = Utf8               ()LA;
  #12 = NameAndType        #9:#10         //  a:LA;
  #13 = Fieldref           #2.#12         //  B.a:LA;
  #14 = Utf8               this
  #15 = Utf8               LB;
  #16 = Utf8               <init>
  #17 = Utf8               ()V
  #18 = NameAndType        #16:#17        //  "<init>":()V
  #19 = Methodref          #4.#18         //  java/lang/Object."<init>":()V
  #20 = Utf8               A
  #21 = Class              #20            //  A
  #22 = Utf8               (I)V
  #23 = NameAndType        #16:#22        //  "<init>":(I)V
  #24 = Methodref          #21.#23        //  A."<init>":(I)V
  #25 = Utf8               ASyntax2$
  #26 = Class              #25            //  ASyntax2$
  #27 = Utf8               MODULE$
  #28 = Utf8               LASyntax2$;
  #29 = NameAndType        #27:#28        //  MODULE$:LASyntax2$;
  #30 = Fieldref           #26.#29        //  ASyntax2$.MODULE$:LASyntax2$;
  #31 = Utf8               A$
  #32 = Class              #31            //  A$
  #33 = Utf8               LA$;
  #34 = NameAndType        #27:#33        //  MODULE$:LA$;
  #35 = Fieldref           #32.#34        //  A$.MODULE$:LA$;
  #36 = NameAndType        #9:#11         //  a:()LA;
  #37 = Methodref          #2.#36         //  B.a:()LA;
  #38 = Utf8               toASyntax2
  #39 = Utf8               (LA;)LA;
  #40 = NameAndType        #38:#39        //  toASyntax2:(LA;)LA;
  #41 = Methodref          #32.#40        //  A$.toASyntax2:(LA;)LA;
  #42 = Utf8               once$extension
  #43 = Utf8               (LA;)I
  #44 = NameAndType        #42:#43        //  once$extension:(LA;)I
  #45 = Methodref          #26.#44        //  ASyntax2$.once$extension:(LA;)I
  #46 = Utf8               A$ASyntax$
  #47 = Class              #46            //  A$ASyntax$
  #48 = Utf8               LA$ASyntax$;
  #49 = NameAndType        #27:#48        //  MODULE$:LA$ASyntax$;
  #50 = Fieldref           #47.#49        //  A$ASyntax$.MODULE$:LA$ASyntax$;
  #51 = Utf8               ASyntax
  #52 = NameAndType        #51:#39        //  ASyntax:(LA;)LA;
  #53 = Methodref          #32.#52        //  A$.ASyntax:(LA;)LA;
  #54 = Utf8               twice$extension
  #55 = NameAndType        #54:#43        //  twice$extension:(LA;)I
  #56 = Methodref          #47.#55        //  A$ASyntax$.twice$extension:(LA;)I
  #57 = Utf8               ASyntax$
  #58 = Utf8               Code
  #59 = Utf8               LocalVariableTable
  #60 = Utf8               LineNumberTable
  #61 = Utf8               SourceFile
  #62 = Utf8               InnerClasses
  #63 = Utf8               RuntimeVisibleAnnotations
  #64 = Utf8               ScalaSig
{
  public A a();
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0       
         1: getfield      #13                 // Field a:LA;
         4: areturn       
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
               0       5     0  this   LB;
      LineNumberTable:
        line 2: 0

  public B();
    flags: ACC_PUBLIC
    Code:
      stack=4, locals=1, args_size=1
         0: aload_0       
         1: invokespecial #19                 // Method java/lang/Object."<init>":()V
         4: aload_0       
         5: new           #21                 // class A
         8: dup           
         9: iconst_1      
        10: invokespecial #24                 // Method A."<init>":(I)V
        13: putfield      #13                 // Field a:LA;
        16: getstatic     #30                 // Field ASyntax2$.MODULE$:LASyntax2$;
        19: getstatic     #35                 // Field A$.MODULE$:LA$;
        22: aload_0       
        23: invokevirtual #37                 // Method a:()LA;
        26: invokevirtual #41                 // Method A$.toASyntax2:(LA;)LA;
        29: invokevirtual #45                 // Method ASyntax2$.once$extension:(LA;)I
        32: pop           
        33: getstatic     #50                 // Field A$ASyntax$.MODULE$:LA$ASyntax$;
        36: getstatic     #35                 // Field A$.MODULE$:LA$;
        39: aload_0       
        40: invokevirtual #37                 // Method a:()LA;
        43: invokevirtual #53                 // Method A$.ASyntax:(LA;)LA;
        46: invokevirtual #56                 // Method A$ASyntax$.twice$extension:(LA;)I
        49: pop           
        50: return        
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
               0      51     0  this   LB;
      LineNumberTable:
        line 5: 0
        line 2: 4
        line 3: 19
        line 4: 36
}