onDispatch demo not compiling

[Shachar via Digitalmars-d-learn]

I'm trying to compile the onDispatch demo program from "The D 
Programming Language" (page 387). At first I had an import 
problem, but I fixed that. Now, however, when I try to call 
"a.do_something_cool", I get an error message saying:

onDispatch.d(43): Error: no property 'do_something_cool' for type 
'onDispatch.A'

Full program follows (into onDispatch.d):
import core.stdc.ctype;
import std.stdio;

string underscoresToCamelCase(string sym) {
     string result;
     result.reserve(sym.length);
     bool makeUpper;
     foreach (c; sym) {
         if (c == '_') {
             makeUpper = true;
         } else {
             if (makeUpper) {
                 result ~= toupper(c);
                 makeUpper = false;
             } else {
                 result ~= c;
             }
         }
     }
     return result;
}

unittest {
     assert(underscoresToCamelCase("hello_world") == "helloWorld");
     assert(underscoresToCamelCase("_a") == "A");
     assert(underscoresToCamelCase("abc") == "abc");
     assert(underscoresToCamelCase("a_bc_d_") == "aBcD");
}

class A {
     auto onDispatch(string m, Args...)(Args args) {
         // return 
mixin("this."~underscoresToCamelCase(m)~"(args)");
     }

     void doSomethingCool(int x, int y) {
         writeln("Do something cool");
     }
}

unittest {
     auto a = new A;
     a.doSomethingCool(5, 6);
     a.do_something_cool(5, 6);
}