C++11 and later define free functions begin, end, empty, etc in namespace std. For most containers these functions invoke the corresponding member function. But for some containers (like valarray) these free functions are overloaded (initializer_list does not have a member begin()). So to iterate over any container free functions should be used and to find functions for container from namespaces other than std ADL should be used:

 template<typename C>
 void foo(C c)
 {
   using std::begin;
   using std::end;
   using std::empty;

   if (empty(c)) throw empty_container();
   for (auto i = begin(c); i != end(c); ++i) { /* do something */ }
 }

Question 1: Am I correct? Are begin and end expected to be found via ADL?

But ADL rules specify that if type of an argument is a class template specialization ADL includes namespaces of all template arguments. And then Boost.Range library comes into play, it defines boost::begin, boost::end, etc. These functions are defined like this:

template< class T >
inline BOOST_DEDUCED_TYPENAME range_iterator<T>::type begin( T& r )
{
    return range_begin( r );
}

If I use std::vector<boost::any> and a Boost.Range I run into trouble. std::begin and boost::begin overloads are ambiguous. That it, I can not write template code that will find a free begin via ADL. If I explicitly use std::begin I expect that any non-std:: container has a member begin.

Question 2: What shall I do in this case?

Rely on the presence of member function? Simplest way.

Ban Boost.Range? Well, algorithms that take container instead of a pair of iterators are convinient. Boost.Range adaptors (containers that lazily apply an algorithm to a container) are also convinient. But if I do not use Boost.Range in my code it still can be used in a boost library (other than Range). This make template code really fragile.

Ban Boost?