Broken contract programing

[Idan Arye via Digitalmars-d]

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