User-defined "opIs"

[Meta via Digitalmars-d]

On Wednesday, 24 September 2014 at 23:08:26 UTC, Adam D. Ruppe 
wrote:
> On Wednesday, 24 September 2014 at 22:48:36 UTC, Meta wrote:
>> Is this supposed to work, and if not, should an enhancement be 
>> made to allow it?
>
> It is not supposed to work - the docs don't list is as 
> overridable (http://dlang.org/expression.html look for 
> "Identity expressions") - and I don't think it should work 
> either. Consider:
>
> class A { opIs() }
>
> if(a is null)
>
> If that were rewritten into if(a.opIs(null))... and a were 
> null, you'd have some trouble. Putting in an automatic null 
> pre-check would start to complicate that is currently a simple 
> operator.

I'm thinking more for structs, which can't be null. The only 
other option right now is to use alias this, but that won't work 
in Proxy's case. For classes, I agree it's tricky... I know we're 
trying to *remove* things from Object, but what if we added:

class Object
{
     static bool opIs(T, U)(inout(T) a, inout(U) b) inout
     {
         //Some implementation
     }
}

And calls to is on an object get rewritten as:

Object.opIs!(typeof(a), typeof(b))(a, b)

So `if (a is null)` becomes `if (Object.opIs(a, null))` only when 
a is a class. When it's a struct, it would just be rewritten as 
`<struct name>.opIs!T(val)`.