functional_type_traits.cpp

#include <type_traits>
#include <iostream>
using namespace std;

auto Valid    = [](auto...x) -> integral_constant<bool,1> {};

template<class F,class...X>
constexpr auto is_callable(F f,X...x) 
-> decltype( f(x...), true_type{} ) {
  return{};
}

constexpr auto is_callable(...) 
-> false_type  {
  return{};
}


template<class F, class X>
auto Fail(F f,X x) 
-> enable_if_t<!is_callable(f,x),true_type> {
  return{};
}


template<class F, class...X>
auto Check(F f, X...x) -> decltype( Valid( f(x)...) ){}

auto Not = [](auto f){
  return [f](auto x){
    return Fail(f,x);
  };
};

auto hasX = [](auto x) -> decltype( x.x(), 0 ) {};


template<class T>
auto doSth(T x) -> decltype(
    Check( hasX(x) ) , 1
) { 
  return 1; 
}

template<class T>
auto doSth(T x) -> decltype(
    Check( Not(hasX)(x) ) , 1
) { 
  return 0; 
}


struct test {
  void x(){}
};


int main() {
  cout<< doSth(1) << doSth(test{}) << endl; //01
}