opBinary failes with templates

[Steven Schveighoffer]

On Wed, 27 Jul 2011 15:48:24 -0400, Asger Dam Hoedt <asgerhoedt at gmail.com>  
wrote:

> Hey
>
> I've very recently started playing around with D, but I seem to have hit  
> my
> head against a bug in dmd. I want to create a Vector class templated with
> its dimension. This works fine until I try to overload the + operator  
> for my
> vector and use it. I then get the error
>
> Error: incompatible types for ((vec) + (vec)): 'Vector!(3)' and  
> 'Vector!(3)'
>
> I've boiled it down to a small example
>
> struct Vector(int D) {
>     float es[D];
>
> public:
>     this(immutable float x, immutable float y, immutable float z) {
>         es[0] = x; es[1] = y; es[2] = z;
>     }
>
>     Vector!(D) opBinary(string s)(immutable Vector!(D) rhs) if (s ==  
> "+") {
>         Vector!(D) ret;
>         for(int i = 0; i < D; ++i)
>             ret.es[i] = es[i] + rhs.es[i];
>         return ret;
>     }
>
> }
>
> alias Vector!(3) Vector3;
>
> void main() {
>     Vector3 vec = Vector3(0,1,2);
>     vec = vec.opBinary!("+")(vec); // This works fine
>     vec = vec + vec; // This line fails miserably
> }
>
> If I replace the template argument in for the argument rhs with 3, then
> everything works, but that's not really a nice solution :)
>
> Is it me trying to use templates in a way they aren't meant to be used  
> or is
> this a bug in dmd? If it is a bug, does anyone have an idea how to solve
> this? I wouldn't mind fixing it in dmd myself, I just need some  
> guidelines.

It is a bug, someone just brought up almost exactly this problem in  
d.learn.

http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D.learn&article_id=28455

The proper workaround (and actually, the cleaner solution) is to use  
Vector and not Vector!D.  Inside a template, the name of the template is  
the same as if you invoked the template with the same template parameters.

If you actually do need a different value for D, I'm not sure what works.

-Steve