Revert "Remove code only needed to detect a pre-4.0 API break."

/// Const traits should never be instantiated.

template <typename Ty> struct ilist_traits<const Ty> {};

namespace ilist_detail {

template <class T> T &make();

/// Type trait to check for a traits class that has a getNext member (as a

/// canary for any of the ilist_nextprev_traits API).

template <class TraitsT, class NodeT> struct HasGetNext {

  typedef char Yes[1];

  typedef char No[2];

  template <size_t N> struct SFINAE {};

  template <class U>

  static Yes &test(U *I, decltype(I->getNext(&make<NodeT>())) * = nullptr);

  template <class> static No &test(...);

public:

  static const bool value = sizeof(test<TraitsT>(nullptr)) == sizeof(Yes);

};

/// Type trait to check for a traits class that has a createSentinel member (as

/// a canary for any of the ilist_sentinel_traits API).

template <class TraitsT> struct HasCreateSentinel {

  typedef char Yes[1];

  typedef char No[2];

  template <class U>

  static Yes &test(U *I, decltype(I->createSentinel()) * = nullptr);

  template <class> static No &test(...);

public:

  static const bool value = sizeof(test<TraitsT>(nullptr)) == sizeof(Yes);

};

/// Type trait to check for a traits class that has a createNode member.

/// Allocation should be managed in a wrapper class, instead of in

/// ilist_traits.

template <class TraitsT, class NodeT> struct HasCreateNode {

  typedef char Yes[1];

  typedef char No[2];

  template <size_t N> struct SFINAE {};

  template <class U>

  static Yes &test(U *I, decltype(I->createNode(make<NodeT>())) * = 0);

  template <class> static No &test(...);

public:

  static const bool value = sizeof(test<TraitsT>(nullptr)) == sizeof(Yes);

};

template <class TraitsT, class NodeT> struct HasObsoleteCustomization {

  static const bool value = HasGetNext<TraitsT, NodeT>::value ||

                            HasCreateSentinel<TraitsT>::value ||

                            HasCreateNode<TraitsT, NodeT>::value;

};

} // end namespace ilist_detail

//===----------------------------------------------------------------------===//

//

/// A wrapper around an intrusive list with callbacks and non-intrusive

      typename base_list_type::const_reverse_iterator const_reverse_iterator;

private:

  // TODO: Drop this assertion and the transitive type traits anytime after

  // v4.0 is branched (i.e,. keep them for one release to help out-of-tree code

  // update).

  static_assert(

      !ilist_detail::HasObsoleteCustomization<TraitsT, value_type>::value,

      "ilist customization points have changed!");

  static bool op_less(const_reference L, const_reference R) { return L < R; }

  static bool op_equal(const_reference L, const_reference R) { return L == R; }