case class extractor works differently when they are handwritten

Hi everyone, before I open a bug report, I would like to have your
opinion on the following :

According to the scala specification, the extractor built by case
classes is the following (scala specification §5.3.2):

def unapply[tps](x: c[tps]) =
if (x eq null) scala.None
else scala.Some(x.xs11, ..., x.xs1k)

For implementation reasons, I want to be able to mimic the behavior of
this extractor on a non-case class. However, my implementation fails
to reproduce the same behavior.

Here is an example of the difference i have:

trait A

sealed trait B[X <: A]{ val x: X }

case class C[X <: A](x: X) extends B[X]

class D[X <: A](val x: X) extends B[X]

object D {
def unapply[X <: A](d: D[X]): Option[X] =
if (d eq None) None
else Some(d.x)
}

def ext[X <: A](b: B[X]) = b match {
case C(x) => Some(x)
case D(x) => Some(x)
case _ => None
}

I have the following warning :

:37: warning: non variable type-argument X in type pattern
D[X] is unchecked since it is eliminated by erasure
case D(x) => Some(x)

Notice the warning occurs only in the D case, not in the case-class
textractor case. Do you have any idea about the cause of the warning /
about what I should do to avoid this warning ?

Furthermore, The AST of object C and D seems to have the same content
for method unapply.

Note: If you want to test it in REPL, the easiest way is:

To activate unchecked warning

scala> :power
scala> settings.unchecked.value = true

To copy above code in paste mode:

scala> :paste
[copy/paste]
[ctrl + D]

Note: this message was first submitted to stack overflow:
http://stackoverflow.com/questions/7008428/difference-between-home-made-...

Best regards
--
Nicolas