Refinement of subtype with itself does not compile

[senier]

The following specification validates:

package Test is

   type Tag is (T1 => 1) with Size => 8;
   type Len is mod 2**8;

   type Packet is
      message
         Tag : Tag;
         Len : Len;
         Val : Opaque
            with Size => 8 * Len;
      end message;

   for Packet use (Val => Packet)                    
      if Tag = T1;

end Test;

The compilation fails:

rflx-test-generic_contains.ads:33:79: "Test_Packet_Context" conflicts with declaration at line 33
gprbuild: *** compilation phase failed