When a) first elem is sequence b) second (and third) elem is(are) void*,
both of conversion ctor and forwarding ctor are candidate since SFINAE
expression has void* type.
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.
