kv.hpp

#include <type_traits>
#include <assert.h>

template
<
  typename Identifier
>
struct Leaf
{
};

template
<
  typename... Identifiers
>
struct KV
:
  Leaf<Identifiers>...
{
  template <typename Identifier>
  using Type = Leaf<Identifier>;
  
  template <typename Identifier>
  static constexpr std::false_type includes(...);
  template <typename Identifier,
            typename=typename std::enable_if_t
                              <std::is_base_of_v<Type<Identifier>, KV>,
                               Type<Identifier>>>
  static constexpr std::true_type includes(bool);
  template <typename Identifier>
  using Includes = decltype(includes<Identifier>(true));
};

/*************************************************/

struct ID {
  struct A {};
  struct B {};
};

using PackA = KV<ID::A>;
using PackB = KV<ID::B>;


int main()
{
  assert( PackA::template Includes<ID::A>::value );
  assert( ! PackA::template Includes<ID::B>::value );
  assert( PackB::template Includes<ID::B>::value );
  assert( ! PackB::template Includes<ID::A>::value );
  return 0;
}