Undefined symbol for, apparently, valid code?

[rikki cattermole via Digitalmars-d-learn]

On 06/07/2017 7:28 AM, unleashy wrote:
> 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!

Templates+classes = require function body.

Why? Templated methods are not virtual, they are final and cannot be 
inherited (so its a little strange that the override is valid).