How about a special null template parameter?

[Timon Gehr via Digitalmars-d]

On 20.08.2016 21:20, Engine Machine wrote:
>>>
>>> That is
>>>
>>> It would be nice to have something like
>>>
>>> alias Type = Type!();
>>> class Type(T...): TypeParent!T if(T.length==1){
>>>     int x;
>>>     static if (T is Dog)
>>>         int y;
>>> }
>>
>> I don't understand how this is related.
>
>
> The only difference is the alias Type = Type!(); Again, D can't do this
> but the point is that it would be nice to have the alias. One can't do
> everything as a "library" solution.
> ...

I see what you are after (but this was not part of the original 
requirements :)   ). I don't think there's a way to make a symbol act as 
both a type and a template.


> Trying to expand your code results in some odd behavior:
>
>
> public template TypeParent(P)
> {
>     import std.traits;
>     alias T = TemplateArgsOf!P;
>     alias Seq(T...) = T;
>     static if (T.length == 0 || is(typeof(T[0]) == typeof(null)))
>     {
>         alias TypeParent = Seq!();
>     }
>     else
>     {
>         alias TypeParent = Seq!(P!(T[0..T.length-1]));
>     }
> }
>
>
> class Type(T...) : TypeParent!(Type!T)
> {
>     int x;
>     static if (T.length >= 1 && T[0] is "Animal")
>     {
>         int y;
>         static if (T.length >= 2 && T[1] is "Dog")
>         {
>             int z;
>             static if (T.length >= 3&& T[2] is "Pug")
>             {
>                 int s;
>             }
>         }
>
>     }
> }
>
>
> void main()
> {
>
>     import std.traits;
>
>     auto a = new Type!("Animal", "Dog", "Pug")();
>     Type!("Animal", "Dog") b = a;
>     Type!("Animal") c = b;
>
>     a.s = 1;
>     b.z = 2;
>     c.y = 3;
> }
>
> b and c are of type P!, not Type!

It seems that this is a compiler bug. Is the problem just with getting a 
string representation of the type?