priority-tag.cpp

#include <type_traits>
#include <cstdio>
template<unsigned N>
struct priority_tag : priority_tag<N-1> {};
template<> struct priority_tag<0> {};

struct sfinae {
template<typename t> static std::enable_if_t<sizeof(t) >= 4> op(t, priority_tag<2> = {}) { printf("foo4\n"); }
template<typename t> static std::enable_if_t<sizeof(t) >= 2> op(t, priority_tag<1> = {}) { printf("foo2\n"); }
template<typename t> static void op(t, priority_tag<0> = {}) { printf("foo1\n"); }
};

template<typename t>
void op(t x) { sfinae::op(x, priority_tag<2>{}); }

int main(void) {
    op(int{});
    op(short{});
    op(char{});
    return 0;
}