template<typename Method, typename>
struct boost::hana::is_default< Method, typename >
Returns whether a tag-dispatched method implementation is a default implementation. 
Given a tag-dispatched method implementation method_impl<T...>, is_default<method_impl<T...>> returns whether method_impl<T...> is a default implementation. Note that if there is no default implementation for the method, then is_default should not be used unless a static assertion saying that "the method is not
implemented" is acceptable.
Example 
#include <algorithm>
#include <iterator>
#include <sstream>
#include <vector>
template <typename T>
struct print_impl : hana::default_ {
    template <typename Stream, typename X>
    static void apply(Stream& out, X 
const& x)
     { out << x; }
};
auto print = [](
auto& stream, 
auto const& x) {
     return print_impl<hana::tag_of_t<decltype(x)>>
::apply(stream, x);
 };
template <typename T>
struct print_impl<
std::vector<T>> {
     template <typename Stream>
    static void apply(Stream& out, std::vector<T> 
const& xs) {
         out << '[';
        std::copy(begin(xs), end(xs), std::ostream_iterator<T>{out, ", "});
        out << ']';
    }
};
static_assert(hana::is_default<print_impl<int>>{}, "");
static_assert(!hana::is_default<print_impl<std::vector<int>>>{}, "");
int main() {
    {
        std::stringstream s;
        print(s, std::vector<int>{1, 2, 3});
     }
    {
        std::stringstream s;
    }
}
Inherits false_type.