disassembled class

  public int go2(org.http4s.client.blaze.Foo$Thing);
    Code:
       0: aload_1       
       1: astore_2      
       2: aload_2       
       3: instanceof    #16                 // class org/http4s/client/blaze/Foo$ThingA
       6: ifeq          48
       9: aload_2       
      10: checkcast     #16                 // class org/http4s/client/blaze/Foo$ThingA
      13: astore_3      
      14: getstatic     #21                 // Field org/http4s/client/blaze/Foo$ThingA$.MODULE$:Lorg/http4s/client/blaze/Foo$ThingA$;
      17: aload_3       
      18: invokevirtual #25                 // Method org/http4s/client/blaze/Foo$ThingA$.unapply:(Lorg/http4s/client/blaze/Foo$ThingA;)Lscala/Option;
      21: astore        4
      23: aload         4
      25: invokevirtual #31                 // Method scala/Option.isEmpty:()Z
      28: ifne          48
      31: aload         4
      33: invokevirtual #35                 // Method scala/Option.get:()Ljava/lang/Object;
      36: invokestatic  #41                 // Method scala/runtime/BoxesRunTime.unboxToInt:(Ljava/lang/Object;)I
      39: istore        5
      41: iload         5
      43: istore        6
      45: goto          93
      48: aload_2       
      49: instanceof    #43                 // class org/http4s/client/blaze/Foo$ThingB
      52: ifeq          96
      55: aload_2       
      56: checkcast     #43                 // class org/http4s/client/blaze/Foo$ThingB
      59: astore        7
      61: getstatic     #48                 // Field org/http4s/client/blaze/Foo$ThingB$.MODULE$:Lorg/http4s/client/blaze/Foo$ThingB$;
      64: aload         7
      66: invokevirtual #51                 // Method org/http4s/client/blaze/Foo$ThingB$.unapply:(Lorg/http4s/client/blaze/Foo$ThingB;)Lscala/Option;
      69: astore        8
      71: aload         8
      73: invokevirtual #31                 // Method scala/Option.isEmpty:()Z
      76: ifne          96
      79: aload         8
      81: invokevirtual #35                 // Method scala/Option.get:()Ljava/lang/Object;
      84: invokestatic  #41                 // Method scala/runtime/BoxesRunTime.unboxToInt:(Ljava/lang/Object;)I
      87: istore        9
      89: iload         9
      91: istore        6
      93: iload         6
      95: ireturn       
      96: new           #53                 // class scala/MatchError
      99: dup           
     100: aload_2       
     101: invokespecial #56                 // Method scala/MatchError."<init>":(Ljava/lang/Object;)V
     104: athrow        

  public int go(scala.util.Either<java.lang.Object, java.lang.Object>);
    Code:
       0: aload_1       
       1: astore_2      
       2: aload_2       
       3: instanceof    #67                 // class scala/util/Left
       6: ifeq          30
       9: aload_2       
      10: checkcast     #67                 // class scala/util/Left
      13: astore_3      
      14: aload_3       
      15: invokevirtual #70                 // Method scala/util/Left.a:()Ljava/lang/Object;
      18: invokestatic  #41                 // Method scala/runtime/BoxesRunTime.unboxToInt:(Ljava/lang/Object;)I
      21: istore        4
      23: iload         4
      25: istore        5
      27: goto          57
      30: aload_2       
      31: instanceof    #72                 // class scala/util/Right
      34: ifeq          60
      37: aload_2       
      38: checkcast     #72                 // class scala/util/Right
      41: astore        6
      43: aload         6
      45: invokevirtual #75                 // Method scala/util/Right.b:()Ljava/lang/Object;
      48: invokestatic  #41                 // Method scala/runtime/BoxesRunTime.unboxToInt:(Ljava/lang/Object;)I
      51: istore        7
      53: iload         7
      55: istore        5
      57: iload         5
      59: ireturn       
      60: new           #53                 // class scala/MatchError
      63: dup           
      64: aload_2       
      65: invokespecial #56                 // Method scala/MatchError."<init>":(Ljava/lang/Object;)V
      68: athrow        
}

file.scala

object Foo {


  sealed trait Thing

  class ThingA(val i: Int) extends Thing
  object ThingA {
    def unapply(thing: ThingA): Option[Int] = Some(thing.i)
  }
  class ThingB(val i: Int) extends Thing
  object ThingB {
    def unapply(thing: ThingB): Option[Int] = Some(thing.i)
  }

  def go2(t: Thing) = t match {
    case ThingA(i) => i
    case ThingB(i) => i
  }

  def go(a: Either[Int, Int]): Int = a match {
    case Left(i) => i
    case Right(i) => i
  }
}