"not an lvalue"

[CrypticMetaphor]

Yes, you were right:

Vector3
{
// ...
/// Binary operator for operator + and -
public Vector3 opBinary(string op) (const ref Vector3 rhs) const if (op 
== "+" || op == "-")
   {
     mixin("return new Vector3(x" ~ op ~ "rhs.x, y" ~ op ~ "rhs.y, z" ~ 
op ~ "rhs.z);");
   }
// ...
}

I removed ref and it worked. I'm a bit confused by this, I'll go read up 
on the ref keyword.