rvalue based copy

[matovitch via Digitalmars-d-learn]

On Monday, 30 March 2015 at 17:14:27 UTC, Adam D. Ruppe wrote:
> On Monday, 30 March 2015 at 17:09:14 UTC, matovitch wrote:
>>(I am gessing ref argument explitly means no rvalue)
>
> That's right. I'd first say don't use ref, just use "const S" 
> and it will work and probably do what you need efficiently.

Yes but you know what they say does it really do a copy of the 
struct or is the compiler smart enougth most of the time to avoid 
copy. (I think it's called return value optimization).
>
> If you do want it to be ref though, rvalues aren't allowed 
> unless you make it "auto ref" which needs to be a template:
>
> // this will work, second set of () makes it a template
> // then auto ref makes it use ref for lvalues and non-ref for 
> rvalues
> // automatially
>     void opAssign()(const auto ref S s)
>     {
>         //...
>     }

Why is this only restricted to templates ?