is_function.hpp

#include <type_traits>

template<int I> struct priority_tag : priority_tag<I - 1> {};
template<> struct priority_tag<0> {};

// Function types here:
template<typename T>
char (&is_function_impl_(priority_tag<0>))[1];

// Array types here:
template<typename T, typename = decltype((*(T*)0)[0])>
char (&is_function_impl_(priority_tag<1>))[2];

// Anything that can be returned from a function here (including
// void and reference types):
template<typename T, typename = T(*)()>
char (&is_function_impl_(priority_tag<2>))[3];

// Classes and unions (including abstract types) here:
template<typename T, typename = int T::*>
char (&is_function_impl_(priority_tag<3>))[4];

template <typename T>
struct is_function
  : std::integral_constant<bool, sizeof(is_function_impl_<T>(priority_tag<3>{})) == 1>
{};