What am I doing wrong here?

[JG]

This isn't code to be used for anything (just understanding).
```d
import std;

struct Delegate(A,B) {
     B function(void* ptr, A a) f;
     void* data;
     B opCall(A a) {
         return f(data,a);
     }
}

auto toDelegate(A, B,S)(S s) {
     static B f(void* ptr, A a) {
         return (*(cast(S*) ptr))(a);
     }
     Delegate!(A,B) ret;
     ret.f=&f;
     ret.data= cast(void*) &s;
     return ret;
}

struct Adder {
     int a;
     int opCall(int b) { return a+b; }
}
auto adder(int a) {
     auto ret = Adder.init;
     ret.a=a;
     return ret;
}

void main() {
     auto g = adder(5);
     g(5).writeln;
     auto d = toDelegate!(int, int)(g);
     d(5).writeln;
     auto a =7;
     auto h = (int b)=>a+b;
     auto d1 = toDelegate!(int,int)(h);
     void* ptr = cast(void*) &h;
     (*cast(int delegate(int)*) ptr)(10).writeln;
     d1(10).writeln;
     h(10).writeln;
}
```