Reduced repro case for Scala 2.10 compiler issue, issue link to be to added.

Test.scala

/*
 Reproduction scenario for Scala 2.10 compiler issue.
 
 To compile: scalac Test.scala
 To execute: 
 
wanamingo:tmp tomer$ scala com.tomergabel.examples.Test
java.lang.ClassFormatError: Duplicate method name&signature in class file com/tomergabel/examples/Derived$$anonfun$defect$1
  at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
	at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
	at com.tomergabel.examples.Derived$class.defect(Test.scala:11)
	at com.tomergabel.examples.Test$.defect(Test.scala:14)
	at com.tomergabel.examples.Test$delayedInit$body.apply(Test.scala:15)
	at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
	at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
	at scala.App$$anonfun$main$1.apply(App.scala:71)
	at scala.App$$anonfun$main$1.apply(App.scala:71)
	at scala.collection.immutable.List.foreach(List.scala:309)
	at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:32)
	at scala.App$class.main(App.scala:71)
	at com.tomergabel.examples.Test$.main(Test.scala:14)
	at com.tomergabel.examples.Test.main(Test.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at scala.tools.nsc.util.ScalaClassLoader$$anonfun$run$1.apply(ScalaClassLoader.scala:71)
	at scala.tools.nsc.util.ScalaClassLoader$class.asContext(ScalaClassLoader.scala:31)
	at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.asContext(ScalaClassLoader.scala:139)
	at scala.tools.nsc.util.ScalaClassLoader$class.run(ScalaClassLoader.scala:71)
	at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:139)
	at scala.tools.nsc.CommonRunner$class.run(ObjectRunner.scala:28)
	at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:45)
	at scala.tools.nsc.CommonRunner$class.runAndCatch(ObjectRunner.scala:35)
	at scala.tools.nsc.ObjectRunner$.runAndCatch(ObjectRunner.scala:45)
	at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:74)
	at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:96)
	at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:105)
	at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala) 
  
  
  Note scalap output for problematic class:
  
wanamingo:tmp tomer$ scalap -cp . 'com.tomergabel.examples.Derived$$anonfun$defect$1'
package com.tomergabel.examples;
final class Derived$$anonfun$defect$1 extends scala.runtime.AbstractPartialFunction with scala.Serializable {
  def this(com.tomergabel.examples.Derived): scala.Unit;
  def applyOrElse(scala.Any, scala.Function1): scala.Any;
  def isDefinedAt(scala.Any): scala.Boolean;
  def isDefinedAt(com.tomergabel.examples.Container): scala.Boolean;
  def applyOrElse(scala.Any, scala.Function1): scala.Any;
}
object Derived$$anonfun$defect$1 {
  final val serialVersionUID: scala.Long;
}


  Work around by qualifiying the return type of Derived.defect:
         protected def defect: PartialFunction[ Container, String ] = { case c: Container => c.v.toString }
 */

package com.tomergabel.examples

case class Container( v: String )

trait Base[ T <: AnyRef ] {
  type UserType = T
	protected def defect: PartialFunction[ UserType, String ]
}

trait Derived extends Base[ Container ] {
  protected def defect = { case c: Container => c.v.toString }
}

object Test extends Derived with App {
	println( defect( Container( "8" ) ) )
}