Determining whether if a function (or variable, or constant) is declared in C++11

As @mattn_jp pointed out on his blog entry, C/C++ allow use of redundant parenthesis in variable declaration. For example, both of the following statements conform to the language specification.

int i = 3; // declares variable i, initialized to 3
int (j) = 4; // declares variable j, initialized to 4

By using this feature, it is possible to create a macro that tests the existence of whatever variable or constant.

The macro: is_defined in the following example is one such implementation.

Compiling and running the code would likely emit is NAN defined? yes. But if you remove #include <cmath> the output will likely change to false, since NAN is a constant defined in cmath.

#include <iostream>
#include <cmath>

struct is_defined_t {
  static bool defined_;
  is_defined_t() { defined_ = false; }
  template <typename T> explicit is_defined_t(const T&) { defined_ = true; }
};

bool is_defined_t::defined_;

#define is_defined(arg) (([]() { is_defined_t(arg); return is_defined_t::defined_; })())

int main(int argc, char **argv)
{
  std::cout << "is NAN defined? " << (is_defined(NAN) ? "yes" : "no") << std::endl;
  return 0;
}

Shortcoming of the example is that the result can be only obtained at runtime. It would be useful if we could get the result at compile time, since once we have such thing, we can get rid of autoconf, CMake, etc.