rep clause in generics

["Jonas Nygren" <ehsjony@ehs.ericsson.se>]

I tried to write a small generic package that represents packed arrays of
some element type:

generic
   type Element_Type is private;
package Gen_Buffers is
   type Index_Type  is new Natural;
   type Buffer_Type is array(Index_Type range <>) of Element_Type;

   pragma Pack (Buffer_Type);
   for Buffer_Type'Component_Size use Element_Type'Size; -- line 9
end Gen_Buffers;

The compiler complained with:

   gen_buffers.ads:9:39: static integer expression required here

I thought that Element_Type'Size was a static integer expression. Am I or
the compiler in error?

/jonas