concepts for c++14

concepts_macro.cpp

// (C) eric niebler
#include <type_traits>

#define CAT_(X, ...) X ## __VA_ARGS__
#define CAT(X, ...) CAT_(X, __VA_ARGS__)

#define RETURN(...) return_t<__VA_ARGS__, std::enable_if_t<RETURN_
#define RETURN_(...) CAT(RETURN_, __VA_ARGS__)>>
#define RETURN_requires

// for real concepts
// #define EXPAND(...) __VA_ARGS__
// #define RETURN(...) __VA_ARGS__ EXPAND

template<class T, class...>
using return_t = T;

template<class T>
auto foo(T) -> RETURN(int)(requires (std::is_integral_v<T>))
{
  return 42;
}

int main()
{
  // foo(""); // compile error
  return foo(1);
}