How do I overload += operator?

[Jack]

I'd like to make this work s += 10 where s is a struct. How can I 
do that?
this isn't working:

     auto ref opAssign(string op, T)(T value)
     if(op == "+")
     {
         m += value;
         return this;
     }

the compiler didn't consider that overload and return:

d.d(34): Error: s is not a scalar, it is a S

full code:

     auto s = S(10);
     writeln(--s);
     s += 5;
     writeln(s);

struct S
{
     int m;

     int opUnary(string op)()
     {
         static if(op == "--")
             return --m;
         else static if(op == "++")
             return ++m;
         else
             static assert(0, "unsupported operator");
     }

     void opBinary(string op, R)(const R rhs)
     if(op == "+")
     {
         m += rhs;
         return this;
     }

     auto ref opAssign(string op, T)(T value)
     if(op == "+")
     {
         m += value;
         return this;
     }
}