Wrong overload resolution protection in implementation of P2363R5

[joaquintides]

boost::container::insert(const_iterator, K&&) is not properly SFINAE-protected. The following is against the develop branch:

#include <boost/container/set.hpp>
#include <functional>

int main()
{
  using set = boost::container::set<int, std::less<>>;

  set s;
  const set cs;
  s.insert(cs.begin(), cs.end());
}

error C2668: 'boost::container::set<int,std::less<void>,void,void>::insert': ambiguous call to overloaded function

I haven't checked boost::container::unordered_set, but chances are it has the same problem.

[igaztanaga]

Many thanks Joaquín!