SFINAE-friendly range_value

[marc.glisse@…]

This code compiles:

#include <boost/range/value_type.hpp>
#include <utility>
#include <iterator>

template<class T>
void f(std::pair<T,T>){}

template<class T>
auto f(T) -> typename std::iterator_traits<typename boost::range_iterator<T>::type>::value_type {}

int main(){
  std::pair<int,int> p;
  f(p);
}

However, if I change the return type to the more readable typename boost::range_value<T>::type, it now fails to compile because the broken typedef in iterator_value is not in the immediate context. The standard noticed that this was useful and made iterator_traits SFINAE-friendly, it would be nice if boost could do the same for the various range meta-functions.