#include <type_traits>

#include <boost/hana/type.hpp>
#include <boost/hana/core/when.hpp>
namespace hana = boost::hana;

struct S {
    template<
        typename T,
        typename = typename std::enable_if_t< (T::value) > > // <-- equivalent?
    S (const T&) { }
};

struct X { static constexpr int value = 0; };
struct Y { static constexpr int value = 1; };

int main () {
    S a (X { }); // <-- must fail
    S b (Y { });
    return 0;
}