REPLExample.scala

//Paste this in the REPL
import language.implicitConversions

trait Exp[+T]
trait Def[+T]

case class Eq[T](a: Exp[T], b: Exp[T]) extends Def[T]
case class Const[T](t: T) extends Exp[T]
class Eq2[T](a: Exp[T], b: Exp[T]) extends Eq[T](a, b) with Exp[T] //This can be defined before or after f1 and f2, it makes no difference.

def f1[T](e: Exp[T]) = e match {
  case Eq(a, b) => 1
  case _ => 0
}
def f2[T](e: Exp[T]) = e match {
  case eq: Eq[u] => 1
  case _ => 0
}

f2(new Eq2(Const(1), Const("Foo")))

//If I actually want to write f2, maybe it's better to make the intent explicit and write:
def f3[T](e: Exp[T]) = e match {
  case eq: Eq[u] with Exp[b] => 1
  case _ => 0
}

REPLOutput.scala

$ ../scala-2.10.0-M7/bin/scala -Xlint -feature
Welcome to Scala version 2.10.0-M7 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_35).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import language.implicitConversions
import language.implicitConversions

scala> 

scala> trait Exp[+T]
defined trait Exp

scala> trait Def[+T]
defined trait Def

scala> 

scala> case class Eq[T](a: Exp[T], b: Exp[T]) extends Def[T]
defined class Eq

scala> case class Const[T](t: T) extends Exp[T]
defined class Const

scala> class Eq2[T](a: Exp[T], b: Exp[T]) extends Eq[T](a, b) with Exp[T]
defined class Eq2

scala> 

scala> def f1[T](e: Exp[T]) = e match {
     |   case Eq(a, b) => 1
     |   case _ => 0
     | }
<console>:13: error: constructor cannot be instantiated to expected type;
 found   : Eq[T]
 required: Exp[?T1] where type ?T1 <: T (this is a GADT skolem)
         case Eq(a, b) => 1
              ^

scala> def f2[T](e: Exp[T]) = e match {
     |   case eq: Eq[u] => 1
     |   case _ => 0
     | }
f2: [T](e: Exp[T])Int

scala> 

scala> f2(new Eq2(Const(1), Const("Foo")))
res0: Int = 1

scala>

scala> def f3[T](e: Exp[T]) = e match {
     |   case eq: Eq[u] with Exp[b] => 1
     |   case _ => 0
     | }
f3: [T](e: Exp[T])Int

scala>