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.
template specialization in C++03 as standard specify it.
This also works on GCC 4.6, 4.8.2 and 4.9 in C++11 and C++03, I needlessly
added this during some debugging where the compiler was telling me to do so.
I shouldn't have followed it's advice, as this breaks compatibility with
other compilers.
attribute_type and attribute_const_type when type is deduced can be
different than when the type is provided.
Indeed when specifying attribute_type and attribute_const_type manually
it's possible to provide a type which isn't const qualified as
attribute_const_type. When deducing the types from the get_expr, a const
and a non const qualified type is taken respectively for attribute_type
and attribute_const_type.
