Cannot implicitly convert derived type

[Jesse Phillips]

On Sunday, 23 February 2014 at 21:06:03 UTC, Frustrated wrote:
>> Why does your WindowsGui violate the iGui contract?
>
> It doesn't. It simply that one can't specify dependencies in D
>
> if (iGui is WindowsGui) then iButton is WindowsButton;
>
> It's not very hard logic but people are not even trying.

I understand what you intend your code to do, but that is still 
breaking the contract which you setup. iGui takes an iButton 
because you defined it that way, to only handle specific types of 
iButton is a breach of contract. The language lets you break 
contracts, but it should not help you do so.