C++ trait to check if class has a member 'type' or not

test.cpp

template <class T>
class has_member_type {
    struct One { char a[1]; };
    struct Two { char a[2]; };

    template <class U>
    static One foo(typename U::type*);

    template <class U>
    static Two foo(...);

public:
    static const bool value = sizeof(foo<T>(nullptr)) == sizeof(One);
};


struct without_type {
};

struct with_type {
    typedef int type;
};

static_assert(!has_member_type<without_type>::value, "");
static_assert(has_member_type<with_type>::value, "");