is_executor template specification fails for inherited executors

[dcbdan]

#include <hpx/config.hpp>
#include <hpx/hpx.hpp>
#include <hpx/hpx_start.hpp>

#include <hpx/include/parallel_executors.hpp>
#include <hpx/include/parallel_algorithm.hpp>

struct my_executor : hpx::parallel::parallel_executor
{};

////////////////////////////////////////////////////////////////////////////////
/* This will only compile if below is uncommented */

//namespace hpx { namespace parallel { HPX_INLINE_NAMESPACE(v3) { namespace detail
//{
//    template <>
//    struct is_executor<my_executor>
//        : std::true_type
//    {};
//}}}}

////////////////////////////////////////////////////////////////////////////////

int hpx_main(int argc, char* argv[])
{
    using hpx::parallel::for_each;
    using hpx::parallel::par;

    my_executor exec;

    std::vector<int> v(100);

    for_each(par.on(exec), v.begin(), v.end(), [](int x){ });

    return hpx::finalize();
}

int main(int argc, char* argv[])
{
    hpx::start(argc, argv);
    return hpx::stop();
}

[hkaiser]

@dcbdan Please see #1693. Does this solve your issue?