Best not to use type_traits undocumented details, these will be moving soon, and use of this header will warn loud and clear if you continue to use it.
Note 1: Forwarding functions are specified as a C++14 constexpr since
std::forward is not a constexpr within C++11.
Note 2: Though I'm not sure why it doesn't compile, some declarations
are specified as a C++14 constexpr or non-constexpr.
Note 3: Boost.Tuple adaptation and TR1-based tuple implementations are
not constexpr.
