trait  Foo {
  val a: Option[String] = None
  def withA(anA:String):Foo = new Foo {
    override val a = Some(anA)
  }
}

class Bar extends Foo

(new Bar()).withA("quux")

java.lang.ClassFormatError: Duplicate method name&signature in class file Main$$$$anon$$2$$Foo$$$$anon$$1
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
	at java.net.URLClassLoader.access$$000(URLClassLoader.java:56)
	at java.net.URLClassLoader$$1.run(URLClassLoader.java:195)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
	at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
	at Main$$$$anon$$2$$Foo$$class.withA((virtual file):7)
	at Main$$$$anon$$2$$Bar.withA((virtual file):12)
	at Main$$$$anon$$2.<init>((virtual file):14)
	at Main$$.main((virtual file):4)
	at Main.main((virtual file))

final <synthetic> class Test$$Foo$$$$anon$$1 extends java.lang.Object with Test$$Foo {
  final <stable> <accessor> def a(): Option = Test$$Foo$$$$anon$$1.this.a;
  private[this] val a: Option = _;
  final <accessor> def a_=(x$$1: Option): Unit = Test$$Foo$$$$anon$$1.this.a = x$$1.$$asInstanceOf[Some]();
  private[this] val a: Some = _;
  override <stable> <accessor> def a(): Some = Test$$Foo$$$$anon$$1.this.a;
  override <stable> <bridge> def a(): Option = Test$$Foo$$$$anon$$1.this.a();
  ...
}