Solving SFINAE issues when you have overlapping conditions

Sometimes we have function templates which we want to use SFINAE on, but some of them have overlapping conditions, creating ambiguity

template<unsigned N, enable_if_t<is_multiple_of<N, 3>>...>
void print_fizzbuzz(){ std::cout << "fizz\n"; }

template<unsigned N, enable_if_t<is_multiple_of<N, 5>>...>
void print_fizzbuzz(){ std::cout << "buzz\n"; }

template<unsigned N, enable_if_t<is_multiple_of<N, 15>>...> // this is ambiguous
void print_fizzbuzz(){ std::cout << "fizzbuzz\n"; }

By using derived-to-base conversions we can create a total ordering for selecting SFINAE overloads.

That is, we resolve ambiguity by using the following inheritance hierarchy:

template<unsigned I> struct choice : choice<I+1>{};

choice<0> has a higher ordering than choice<1>, and we can therefore use choice<0> as a function parameter to make is_multiple_of<N, 15> a better overload, thereby resolving the ambiguity.

The complete fizzbuzz example:

#include <type_traits>

#include <iostream>

template<class C, class T = int>
using enable_if_t = typename std::enable_if<C::value, T>::type;

template<int N, int M>
struct is_multiple_of : std::integral_constant<bool, N % M == 0>{};

//-------------------------------

template<unsigned I> struct choice : choice<I+1>{};
template<> struct choice<10>{}; // suitably high terminating condition

struct otherwise{ otherwise(...){} };

struct select_overload : choice<0>{};

//-------------------------------

template<unsigned N, enable_if_t< is_multiple_of<N, 15> >...>
void print_fizzbuzz(choice<0>) { std::cout << "fizzbuzz\n"; }

template<unsigned N, enable_if_t< is_multiple_of<N, 3> >...>
void print_fizzbuzz(choice<1>) { std::cout << "fizz\n"; }

template<unsigned N, enable_if_t< is_multiple_of<N, 5> >...>
void print_fizzbuzz(choice<2>) { std::cout << "buzz\n"; }

template<unsigned N>
void print_fizzbuzz(otherwise){ std::cout << N << "\n"; }

template<unsigned N = 1>
void do_fizzbuzz()
{
    print_fizzbuzz<N>(select_overload{});
    do_fizzbuzz<N+1>();
}

template<>
void do_fizzbuzz<50>()
{
    print_fizzbuzz<50>(select_overload{});
}

//-------------------------------

int main()
{
    do_fizzbuzz();
}

This excellent technique by Xeo, as described here