DIP66 - Multiple alias this

[Daniel N via Digitalmars-d]

On Wednesday, 15 October 2014 at 02:46:05 UTC, Dicebot wrote:
> On Tuesday, 14 October 2014 at 12:33:50 UTC, IgorStepanov wrote:
>> This code tell that C is subtype of A and C is subtype of B.
>> User can use this fact in his code:
>> void foo(B);
>>
>> C c = new C;
>> foo(c); //Ok.
>> Of course, we shouldn't allow user to cast c to int:
>> int i = c; //wrong
>> However, user can explicitly cast c to his subtype, which is 
>> convertable to int:
>> int i = cast(B)c; //Ok
>> Summarizing, I disagree with suggestion disallow this code at 
>> type semantic stage.
>
> I agree. It will also make possible to break already working 
> disambugation of `foo(c)` kind but adding new `alias this` to 
> one of subtypes independently. That sounds annoying.

I guess the best part of D is, you have the means to fix anything 
you disagree with yourself... I can add a static assert to my 
class and be happy.

I have another idea, we could define that the shortest conversion 
chain wins, analogous to type promotions, that makes it possible 
to contain the issue inside C.

class C
{
   A a;
   B b;

   int disambiguate_int()
   {
     return a;
   }
   alias a this;
   alias b this;
   alias disambiguate_int this;
   static assert(__traits(compiles, {int _ = C.init;}), "Ambiguous 
alias this");
}

i.e. this assert should pass.