How do I determine if a type is callable with only const references?

Question

I want to write a C++ metafunction is_callable that defines value to be true, if and only if the type F has the function cal

Answer 1

(with apologies to Kerrek for using his answer as a starting point)

EDIT: Updated to handle types without any operator() at all.

#include 

template 
struct Callable
{
private:
  static int tester[1];
  typedef char                      yes;
  typedef struct { char array[2]; } no;

  template 
  static char sfinae(decltype(std::declval()(std::declval())) (G::*pfn)(Brg)) { return 0; }

  template 
  static char sfinae(decltype(std::declval()(std::declval())) (G::*pfn)(Brg) const) { return 0; }

  template 
  static yes test(int (&a)[sizeof(sfinae(&G::operator()))]);

  template 
  static no test(...);

public:
  static bool const value = sizeof(test(tester)) == sizeof(yes);
};

struct Foo
{
  int operator()(int &) { return 1; }

};

struct Bar
{
  int operator()(int const &) { return 2; }
};

struct Wazz
{
  int operator()(int const &) const { return 3; }
};

struct Frob
{
  int operator()(int &) { return 4; }
  int operator()(int const &) const { return 5; }
};

struct Blip
{
  template
  int operator()(T) { return 6; }
};

struct Boom
{

};

struct Zap
{
  int operator()(int) { return 42; }
};

#include 
int main()
{
  std::cout << "Foo(const int &):  " << Callable::value << std::endl
            << "Foo(int &):        " << Callable::value << std::endl
            << "Bar(const int &):  " << Callable::value << std::endl
            << "Bar(int &):        " << Callable::value << std::endl
            << "Zap(const int &):  " << Callable::value << std::endl
            << "Zap(int&):         " << Callable::value << std::endl
            << "Wazz(const int &): " << Callable::value << std::endl
            << "Wazz(int &):       " << Callable::value << std::endl
            << "Frob(const int &): " << Callable::value << std::endl
            << "Frob(int &):       " << Callable::value << std::endl
            << "Blip(const int &): " << Callable::value << std::endl
            << "Blip(int &):       " << Callable::value << std::endl
            << "Boom(const int &): " << Callable::value << std::endl
            << "Boom(int&):        " << Callable::value << std::endl;
}

Demo: http://ideone.com/T3Iry