DIP66 - Multiple alias this

[IgorStepanov via Digitalmars-d]

On Friday, 10 October 2014 at 20:47:45 UTC, Steven Schveighoffer 
wrote:
> On 10/10/14 1:09 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.
>
> This part:
>
> void test()
>     {
>         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.
>
> I think might be wrong. There is a lot of code out there that 
> says, e.g.:
>
> void foo(T)(T t) if(is(T : U))
> {
>   U u = t;
>   ...
> }
>
> Which will now create an error in the wrong place. IMO, the 
> 'is' test should also fail.
>
> -Steve

I thought exactly about this using case.

See:
You have a struct like this in first place:
     struct A
     {
         int i;
         alias i this;
     }

     struct C
     {
         A a;
         string s;
         alias a this;
         alias s this;
     }

And you have a template function in second place:
void foo(T)(T t) if(is(T : int))
{
...
}

void foo(T)(T t) if(is(T : string))
{
...
}


And you have the code it third place:
C c;
foo(c); //Error: what do you mean: foo!(T : string) or foo!(T : 
int)

Now, someone (A developer) changed the A definition:

     struct A
     {
         int i;
         alias i this;
     }

     struct B
     {
         int i;
         alias i this;
     }

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

And now, you code mystically start to works.
Attention: Infusing in one place conflict resolves conflict in 
another place.
It is danger, I think.