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.
As the BOOST_FUSION_ADAPT_STRUCT_FILLER* macros were changed to handle type
deduction, this breaks the usage that BOOST_FUSION_DEFINE_STRUCT makes from
them.
That is BOOST_FUSION_DEFINE_STRUCT_IMPL expects a simple tuple, while we
now provide a tuple containing the size of a nested tuple which either has
or not the type provided. But in the case of the DEFINE* macros it would be
a non-sense to support this kind of API as a field being defined cannot be
deduced. :)
