Mixin Templates and Operators

[francesco.andreetto]

I have two structs, point and vec, in which I want to implement 
some arithmetic operations.
The operations are:

```
point - point = vec
point + vec = point
```

Using mixin templates the code compiles but calling the 
operations in the main causes an "incompatible type" Error:

```d
mixin template sum(T, R) {
    R opBinary(string op)(T rhs) const
    if (op == "+"){
       return R(x + rhs.x, y + rhs.y, z + rhs.z);
    }
}

mixin template diff(T, R) {
    R opBinary(string op)(T rhs) const
    if (op == "-") {
       return R(x - rhs.x, y - rhs.y, z - rhs.z);
    }
}

struct point{
    float x, y, z;
    mixin diff!(point, vec);
    mixin sum!(vec, point);
}

struct vec{
    float x, y, z;
}

void main(){
    point p1 = {1,2,3}, p2 = {5,6,7};
    vec v1 = p1-p2;

    vec v2 = {2,-1,0};
    point p3 = p1+v2;
}
```
The output of `dub run` is

```
Performing "debug" build using /usr/bin/dmd for x86_64.
query ~master: building configuration "application"...
source/app.d(27,13): Error: incompatible types for `(p1) - (p2)`: 
both operands are of type `point`
source/app.d(30,15): Error: incompatible types for `(p1) + (v2)`: 
`point` and `vec`
/usr/bin/dmd failed with exit code 1.
```

I tried to implement a single template:

```d
mixin template sumDiff(T, R){
    R opBinary(string op)(T rhs) const
    if (op == "+" || op == "-"){
       return mixin("R(x " ~ op ~ " rhs.x, y " ~ op ~ "rhs.y, z " 
~ op ~ " rhs.z)");
    }
}
```

but the same error occurs.

If I don't use the mixin templates and, instead, I overload the 
operations into the structures everything works.

```d
struct point{
    float x, y, z;

    vec opBinary(string op)(point rhs) const
    if (op == "-") {
       return vec(x - rhs.x, y - rhs.y, z - rhs.z);
    }

    point opBinary(string op)(vec rhs) const
    if (op == "+") {
       return point(x + rhs.x, y + rhs.y, z + rhs.z);
    }
}

struct vec{
    float x, y, z;
}

void main(){
    point p1 = {1,2,3}, p2 = {5,6,7};
    vec v1 = p1-p2;

    vec v2 = {2,-1,0};
    point p3 = p1+v2;
}
```

Am I doing something wrong or it is impossible to use mixin 
templates like that?