Is there a way to test whether a C++ class has a default constructor (other than compiler-provided type traits)?

Question

Traits classes can be defined to check if a C++ class has a member variable, function or a type (see here).

Curiously, the ConceptTraits do not include traits to che

Answer 1

After shedding much blood, sweat, and tears, I finally found a way that works on every compiler I tried:

template struct is_default_constructible;

template<> struct is_default_constructible
{
protected:
    // Put base typedefs here to avoid pollution
    struct twoc { char a, b; };
    template struct test { typedef char type; };
public:
    static bool const value = false;
};
template<> struct is_default_constructible<>::test { typedef twoc type; };

template struct is_default_constructible : is_default_constructible<>
{
private:
    template static typename test::type sfinae(U*);
    template static char sfinae(...);
public:
    static bool const value = sizeof(sfinae(0)) > 1;
};