Code fails with linker error. Why?

[eles via Digitalmars-d-learn]

On Monday, 6 October 2014 at 11:54:56 UTC, John Colvin wrote:
> On Monday, 6 October 2014 at 10:10:04 UTC, eles wrote:
>> On Saturday, 4 October 2014 at 15:29:57 UTC, John Colvin wrote:
>>> On Saturday, 4 October 2014 at 11:19:52 UTC, ketmar via 
>>> Digitalmars-d-learn wrote:
>>>> On Sat, 04 Oct 2014 11:01:28 +0000
>>>> John Colvin via Digitalmars-d-learn 
>>>> <digitalmars-d-learn at puremagic.com>
>>>> wrote:
>>>>
>>>>> On Saturday, 4 October 2014 at 10:38:32 UTC, ketmar via 
>>>>> Digitalmars-d-learn wrote:
>>>>> > On Sat, 04 Oct 2014 10:27:16 +0000
>>>>> > John Colvin via Digitalmars-d-learn 
>>>>> > <digitalmars-d-learn at puremagic.com>
>>>>> > wrote:

> I don't quite follow. Example?

Well, in the OP example, imagine that I was trying to compile 
this module along with another one that simply happened to have a 
method defined in a way that the linker would have find it.

I would have compiled with:

dmd app.d app2.d

and be unaware what bug I have introduced because I forgot do 
declare formula() as abstract in the first class.