Welcome to Scala version 2.11.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_05).
Type in expressions to have them evaluated.
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scala> :paste
// Entering paste mode (ctrl-D to finish)

object A{
  final val a = "a"
  final val b = "b"
  def foo(x: a.type) = x
  def foo(x: b.type) = x
}

// Exiting paste mode, now interpreting.

java.lang.ClassFormatError: Duplicate method name&signature in class file A$
  at java.lang.ClassLoader.defineClass1(Native Method)
  at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
  at scala.reflect.internal.util.AbstractFileClassLoader.findClass(AbstractFileClassLoader.scala:87)
  at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
  at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
  at java.lang.Class.getDeclaredMethods0(Native Method)
  at java.lang.Class.privateGetDeclaredMethods(Class.java:2688)
  at java.lang.Class.privateGetPublicMethods(Class.java:2814)
  at java.lang.Class.getMethods(Class.java:1602)
  at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.evalMethod(IMain.scala:800)
  at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:734)
  at scala.tools.nsc.interpreter.IMain$Request.loadAndRun(IMain.scala:986)
  at scala.tools.nsc.interpreter.IMain$WrappedRequest$$anonfun$loadAndRunReq$1.apply(IMain.scala:593)
  at scala.tools.nsc.interpreter.IMain$WrappedRequest$$anonfun$loadAndRunReq$1.apply(IMain.scala:592)
  at scala.reflect.internal.util.ScalaClassLoader$class.asContext(ScalaClassLoader.scala:31)
  at scala.reflect.internal.util.AbstractFileClassLoader.asContext(AbstractFileClassLoader.scala:19)
  at scala.tools.nsc.interpreter.IMain$WrappedRequest.loadAndRunReq(IMain.scala:592)
  at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:524)
  at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:520)
  at scala.tools.nsc.interpreter.ILoop.interpretCode$1(ILoop.scala:707)
  at scala.tools.nsc.interpreter.ILoop.pasteCommand(ILoop.scala:721)
  at scala.tools.nsc.interpreter.ILoop$$anonfun$standardCommands$10.apply(ILoop.scala:220)
  at scala.tools.nsc.interpreter.ILoop$$anonfun$standardCommands$10.apply(ILoop.scala:220)
  at scala.tools.nsc.interpreter.LoopCommands$LineCmd.apply(LoopCommands.scala:62)
  at scala.tools.nsc.interpreter.ILoop.command(ILoop.scala:661)
  at scala.tools.nsc.interpreter.ILoop.processLine(ILoop.scala:433)
  at scala.tools.nsc.interpreter.ILoop.loop(ILoop.scala:450)
  at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply$mcZ$sp(ILoop.scala:868)
  at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:854)
  at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:854)
  at scala.reflect.internal.util.ScalaClassLoader$.savingContextLoader(ScalaClassLoader.scala:95)
  at scala.tools.nsc.interpreter.ILoop.process(ILoop.scala:854)
  at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:74)
  at scala.tools.nsc.MainGenericRunner.run$1(MainGenericRunner.scala:87)
  at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:98)
  at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:103)
  at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)