Is it possible to specialize a template definition based on the existence of a nested typedef of a template type parameter?

Question

I have a template, template class wrapper, that I would like to specialize based on the existence of typename T::context_type. If

Answer 1

Using @K-ballo's answer, I wrote the following:

namespace detail {
BOOST_MPL_HAS_XXX_TRAIT_DEF(context_type)
}

template 
class wrapper
{
public:
    wrapper() {
        std::cout << "T::context_type does not exist." << std::endl;
    }
};

template 
class wrapper >::type>
{
public:
    typedef typename T::context_type context_type;

private:
    context_type ctx;

public:
    wrapper(context_type ctx_)
        : ctx(ctx_)
    {
        std::cout << "T::context_type exists." << std::endl;
    }
};

Now, the sample code compiles and outputs:

T::context_type exists.
T::context_type does not exist.