Virtual opBinary in interface

[mzfhhhh]

On Thursday, 19 December 2024 at 18:49:28 UTC, sfp wrote:
> Subject lines says it all, I think... The choice to make binary 
> operators implementable only via this `opBinary` template means 
> it's unclear how to get virtual operators on an interface. 
> E.g., this toy example *does* compile:
> ```
> interface Scalar {
>   Scalar opBinary(string op)(Scalar rhs); // wrong
> }
>
> class Int : Scalar {
>   int i;
>   this(int i) { this.i = i; }
>   Int opBinary(string op)(Int rhs) if (op == "+") {
>     return new Int(i + this.i);
>   }
> }
>
> void main() {
>   Scalar one = new Int(1);
>   Scalar two = one + one;
> }
> ```


The template methods in the interface need to be implemented 
within the interface.

https://dlang.org/spec/interface.html#method-bodies

```d
interface Scalar {
     Scalar opBinary(string op)(Scalar rhs) if (op == "+") {
         return add(rhs);
     }

     Scalar add(Scalar rhs);
}


class Int : Scalar {
     int i;
     this(int i) { this.i = i; }

     Scalar add(Scalar rhs) {
     return new Int((cast(Int)rhs).i + this.i);
     }
}

void main() {
     Scalar one = new Int(1);
     Scalar two = one + one;
}
```