Undefined symbol for, apparently, valid code?

[unleashy via Digitalmars-d-learn]

Hello. I am trying to compile this:

---
module asd.asd;

abstract class Asd
{
     void opCall(Args...)(Args args);
}

@system unittest
{
     class Foo : Asd
     {
         override void opCall(Args...)(Args args)
         {
             /* nothing */
         }
     }

     Asd a = new Foo();

     a(1, 2, 3);
}
---

This file is under source/asd/asd.d and I'm compiling with `dmd 
-unittest -main -ofasd.exe source\asd\asd.d -m32 -g` under 
Windows x64. For some reason, after the successful compilation 
step, the linker then complains:

---
OPTLINK (R) for Win32  Release 8.00.17
Copyright (C) Digital Mars 1989-2013  All rights reserved.
http://www.digitalmars.com/ctg/optlink.html
asd.obj(asd)
  Error 42: Symbol Undefined 
_D3asd3asd3Asd17__T6opCallTiTiTiZ6opCallMFiiiZv
Error: linker exited with status 1
---

Am I doing something wrong, or is this a linker bug?

Thanks!