DIP66 - Multiple alias this

[IgorStepanov via Digitalmars-d]

On Monday, 13 October 2014 at 00:54:13 UTC, Steven Schveighoffer 
wrote:
> On 10/12/14 7:16 PM, IgorStepanov wrote:
>> On Sunday, 12 October 2014 at 23:02:13 UTC, Steven 
>> Schveighoffer wrote:
>>> On 10/10/14 6:10 PM, IgorStepanov wrote:
>
>>>> You can write foo(c.shadow); This isn't hard.
>>>> Ok, I understood you, let's listen to what others say
>>>
>>> Right, you can get around it.
>>>
>>> But the issue here is, that I feel like is(T: U) means (from 
>>> dlang.org):
>>>
>>> is ( Type : TypeSpecialization )
>>> The condition is satisfied if Type is semantically correct 
>>> and it is
>>> the same as or can be implicitly converted to 
>>> TypeSpecialization.
>>>
>>> This means is(C : int) should indicate that C can implicitly 
>>> convert
>>> to int. But in your DIP, it does not. I think this is 
>>> incorrect.
>
>>
>> Hmm. I've written case (my previous post), when returning 
>> false from
>> is(T: S), where T has many pathes to S is dangerous.
>
> OK, I didn't understand your case before, but I just got it.
>
> I understand what you mean, but this isn't anything new -- one 
> can cause weird problems by creating diamond-pattern interfaces 
> also. I do not actually think it is dangerous, because one 
> would not leave an error call in their code. So for a future 
> change to a library to "mystically" make a function start 
> working is not a danger, because said code wasn't sitting there 
> broken in the first place.
>
> I will note, that for diamond problem interfaces, the compiler 
> seems to take a different track than your DIP:
>
> interface A {}
> interface B : A {}
> interface C : A {}
>
> class X : B, C {}
>
> static assert(is(X : A));
>
> void main()
> {
>     A a = new C; // works, not sure if it's B.A or C.A
> }
>
> I know this is a different problem -- we aren't pointing at two 
> different concrete implementations.
>
> -Steve

This is fundamentally different situation: interfaces haven't a 
state, thus don't care what interface will be getted: B.C or C.C. 
Moreover, we can think that we have only one base C (like virtual 
inherited class in C++).
Alias this case requires a completely different approach.