How to refer to user defined literal operator inside a namespace?

Consider the following:

#include <iostream>

namespace X
{
    void operator ""_test(unsigned long long x)
    {
        std::cout << x;
    }
}

int main()
{
    using namespace X;
    10_test;
    // 10_X::test; /* doesn't work */
}

I can refer to the user defined literal operator inside the namespace X by an explicit using namespace X;. Is there any way of referring to the literal operator without explicitly including the namespace? I tried the

10_X::test;

but of course doesn't work as the parser believes X refers to the name of the operator.

X::operator ""_test(10)

works but it's clumsy.