Broken contract programing

[via Digitalmars-d]

On Wednesday, 13 May 2015 at 18:32:17 UTC, Idan Arye wrote:
> On Wednesday, 13 May 2015 at 17:47:27 UTC, Ola Fosheim Grøstad 
> wrote:
>> On Wednesday, 13 May 2015 at 14:54:05 UTC, Benjamin Thaut 
>> wrote:
>>> On Wednesday, 13 May 2015 at 12:54:52 UTC, Timon Gehr wrote:
>>>> This will accept the same arguments as Base. Only one 
>>>> in-contract in the inheritance chain must pass in order for 
>>>> the call to go through.
>>>
>>> But wasn't exactly that the problem about the current 
>>> implementation? Shouldn't all 'in' contracts in the chain be 
>>> checked?
>>
>> The topic is non-trivial. Dart has even deliberately picked an 
>> unsound inheritance typing-model because it is more useful. 
>> Consider the following scenario:
>>
>> You have a superclass Shelter, then you create subclass 
>> DogShelter. The Shelter has a virtual function add_pet(Animal 
>> x) . So clearly the DogShelter will have to accept the _type_ 
>> Animal in its add_pet specialization because of the type 
>> system.
>>
>> That does not mean it is not a bug if you bring a Cat to the 
>> DogShelter. It merely means that you cannot catch such bugs 
>> with the type system alone. The DogShelter might sit behind 
>> another infrastructure that is supposed to prevent non-Dog 
>> animals from entering.
>>
>> So it makes sense to not conflate invariants/contracts with 
>> the type system.
>
> http://en.wikipedia.org/wiki/Covariance_and_contravariance_%28computer_science%29

Yes… and your point is?