begin() and end() free function overload on template

Question

I have a templated class, Iterable; for which I want to overload the begin() and end() free functions. It stores data as a vector of unique_ptr, but the interface uses boost::indirect_iterator for convenience.

My code builds and runs under CLang-3.5, but I tried on g++-4.9 and it did not. But I do not know why ? (And which compiler has the right behaviour).

template<typename T>
using SimpleVec = std::vector<T, std::allocator<T>>;

template <typename T,
          template <typename> class Container = SimpleVec,
          class String = std::string>
class Iterable
{
        template <typename friendT,
                  template <typename> class friendContainer,
                  class friendString>
        friend boost::indirect_iterator<typename friendContainer<std::unique_ptr<friendT>>::iterator>
            begin(Iterable<friendT, friendContainer, friendString>& i);

        template <typename friendT,
                  template <typename> class friendContainer,
                  class friendString>
        friend boost::indirect_iterator<typename friendContainer<std::unique_ptr<friendT>>::iterator>
            end(Iterable<friendT, friendContainer, friendString>& i);
};

And the free functions :

template <typename T,
          template <typename> class Container = SimpleVec,
          class String = std::string>
boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>
    begin(Iterable<T, Container, String>& i)
{
    return boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>(begin(i._c));
}

template <typename T,
          template <typename> class Container = SimpleVec,
          class String = std::string>
boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>
    end(Iterable<T, Container, String>& i)
{
    return boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>(end(i._c));
}

On g++, the error is :

../../API/net/session/ClientSession.h:83:29: error: call of overloaded 'begin(GroupManager&)' is ambiguous
      for(auto& grp : groups())
                             ^
../../API/net/session/ClientSession.h:83:29: note: candidates are:
In file included from ../../API/net/session/../permission/full/PermissionManager.h:5:0,
                 from ../../API/net/session/Session.h:3,
                 from ../../API/net/session/ClientSession.h:2,
                 from ../../API/net/session/ClientSessionBuilder.h:2,
                 from ../client/main.cpp:2:
../../API/net/session/../permission/full/../../Iterable.h:22:4: note: boost::indirect_iterator<typename friendContainer<std::unique_ptr<friendT> >::iterator> begin(Iterable<friendT, friendContainer, friendString>&) [with friendT = Group; friendContainer = SimpleVec; friendString = std::basic_string<char>; T = Permission; Container = SimpleVec; String = std::basic_string<char>; typename friendContainer<std::unique_ptr<friendT> >::iterator = __gnu_cxx::__normal_iterator<std::unique_ptr<Group, std::default_delete<Group> >*, std::vector<std::unique_ptr<Group, std::default_delete<Group> >, std::allocator<std::unique_ptr<Group, std::default_delete<Group> > > > >]
    begin(Iterable<friendT, friendContainer, friendString>& i);
    ^
../../API/net/session/../permission/full/../../Iterable.h:142:2: note: boost::indirect_iterator<typename Container<std::unique_ptr<_Tp> >::iterator> begin(Iterable<T, Container, String>&) [with T = Group; Container = SimpleVec; String = std::basic_string<char>; typename Container<std::unique_ptr<_Tp> >::iterator = __gnu_cxx::__normal_iterator<std::unique_ptr<Group, std::default_delete<Group> >*, std::vector<std::unique_ptr<Group, std::default_delete<Group> >, std::allocator<std::unique_ptr<Group, std::default_delete<Group> > > > >]
  begin(Iterable<T, Container, String>& i)
  ^

So it looks like g++ sees the friend declaration as another function ?

Answer 1

I finally managed to find an answer (which is much more readable) thanks to @sehe's answer and this cppreference page.

template <typename T,
          template <typename> class Container = SimpleVec,
          class String = std::string>
class Iterable
{
        friend boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>
            begin(Iterable& i)
        {
            return boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>(begin(i._c));
        }

        friend boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>
            end(Iterable& i)
        {
            return boost::indirect_iterator<typename Container<std::unique_ptr<T>>::iterator>(end(i._c));
        }
};