DIP66 - Multiple alias this

[IgorStepanov via Digitalmars-d]

On Friday, 10 October 2014 at 22:05:18 UTC, Timon Gehr wrote:
> On 10/10/2014 07:09 PM, IgorStepanov wrote:
>> I've created DIP for my pull request.
>> DIP: http://wiki.dlang.org/DIP66
>> PR: https://github.com/D-Programming-Language/dmd/pull/3998
>>
>> Please, comment it.
>
> - "C c;
> int i = c; //Error: c.a.i vs c.b.i
>
> static assert(is(C : int)); //Ok, because C is subtype of int 
> anyway."
>
> So now we can have 'subtypes' whose instances cannot be stored 
> in variables of the 'base type'?

C++ allowed subtypes, which can not be casted to the base type 
(inheritance of two identical, non-virtual base classes).
Ok, I've wrote my position, understood your and wait the decision 
of the arbitrator:)

> Such behaviour is inconsistent with both the reference 
> implementation and the documentation (and the two happen to be 
> mutually inconsistent on how 'is(:)' should behave as well. :o) 
> )
>
>
> - "The following pseudo-code illustrates this: [...] Finally, 
> if resultSet contains only one candidate, the compiler will 
> accept it."
>
> This process might very well never terminate but it could 
> terminate in more cases if it did something better than the 
> naive brute-force search. I.e. either report cycles or don't 
> keep exploring around cycles, but just looping indefinitely on 
> cycles like the following is IMO not a good course of action:
>
> struct S{
>     alias get this;
>     T get(){ return T.init; }
> }
> struct T{
>     alias get this;
>     S get(){ return S.init; }
>     int x;
>     alias x this;
> }
>
> void main(){
>     S s;
>     int x=s;
> }

This case described in DIP below.
Recursion tree will be like:
s.get
    s.get.get ->return, because T is already visited
s.x -> win

> Furthermore, the following code compiles now, but doesn't under 
> the approach described in the DIP. Is this an actual regression 
> your pull introduces or is there a bug in the pseudocode?:
>
> class A{
>     alias x this;
>     int x;
> }
>
> class B: A{
>     alias y this;
>     int y;
> }
>
> void main(){
>     int x = new B();
> }
>
> The same issue also needs to be considered if A and B are 
> structs instead and B has an additional alias this to an A (the 
> solution might also be part of a fix for the cycle issue).
>
> - "If resultSet contains more then one candidates, the compiler 
> raises an error."

struct A
{
    short s;
    alias s this;
}

struct B
{
    int i;
    alias i this;
}

struct C
{
    A a;
    B b;
    alias a this;
    alias b this;
}

long l = C(); //What do you suggest?