Concepts.cpp

#include <iostream>
#include <cstdlib>
#include <type_traits>

#include "donot-change.h"

struct X
{
};


template <class T>
concept printable = requires (T x) { ::std::cout << x; };

template<typename T>
    requires (!printable<T>)
void print(const T&)
{
    ::std::cout << "unknown" << ::std::endl;
}

int main()
{
    print(1);
    print(nullptr);
    print(X());
}
// https://wandbox.org/permlink/yznTfVoLiDd3deC9

donot-change.h

template<typename T>
void print(const T& val)
{
    ::std::cout << val << ::std::endl;
}

SFINAE.cpp

#include <iostream>
#include <cstdlib>
#include <type_traits>

#include "donot-change.h"

struct X
{
};


void print(...)
{
    ::std::cout << "unknown" << ::std::endl;
}

template<typename T>
void print(const T& val, typename std::enable_if<std::is_same<decltype((::std::cout << std::declval<T>()), int()), int>::value>::type* = nullptr)
{
    ::std::cout << val << ::std::endl;
}

#if __GNUC__ < 8
template<>
void print(const ::std::nullptr_t&)
{
    ::std::cout << "nullptr" << ::std::endl;
}
#endif


int main()
{
    print(1);
    print(nullptr);
    print(X());
}
// https://wandbox.org/permlink/5Yoc4b6Px7V89r17