this was spotted by a new GCC warning -Wunused-function
and I must admit, GCC is right here: an externally not visible
function in an anonymous namespace is not what I'd expect to be
picked up by ADL. It is rather weird that the metaprogramming
trait worked at all.
Note that the function is intentionally declared only, never defined.
We want a linker error in case boost::hash ever attempts to
use this 'deliberately ill-definded' catch-all.
NOTE: this header contains a potentially dangerous, temporary workaround
to defeat the static assertion in the default implementation of std::hash,
as shipped with GCC 4.7.x
This assertion turns out to be detrimental all kinds of metaprogramming
based solutions, since it defeats SFINAE. It is expected to be removed
in GCC 4.8
