[Bug libstdc++/109751] New: boost interface_interface fails concept check starting in gcc-13

["ldalessandro at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109751

            Bug ID: 109751
           Summary: boost interface_interface fails concept check starting
                    in gcc-13
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldalessandro at gmail dot com
  Target Milestone: ---

The following example is reduced from
https://github.com/boostorg/stl_interfaces/blob/boost-1.82.0/example/node_iterator.cpp.
It compiles fine in gcc-12 but fails a concept check in gcc-13.


  #include <boost/stl_interfaces/iterator_interface.hpp>

  template<typename T>
  struct iterator : boost::stl_interfaces::iterator_interface<iterator<T>, 
  std::forward_iterator_tag, T>
  {
    using base_type = boost::stl_interfaces::iterator_interface<iterator<T>,
std::forward_iterator_tag, T>;
    using base_type::operator++;

    T& operator*() const;
    iterator& operator++();
    bool operator==(iterator) const;
  };

  static_assert(std::incrementable<iterator<int>>);

Live: https://godbolt.org/z/6qsx1narj

The error tells us that this requires clause changes from false to true.

      friend constexpr bool operator<(D lhs, D rhs)
        requires std::equality_comparable<D> {
          return (lhs - rhs) < typename D::difference_type(0);
        }

https://github.com/boostorg/stl_interfaces/blob/boost-1.82.0/include/boost/stl_interfaces/iterator_interface.hpp#L779

I don't think that requires clause makes any sense, but I also don't know why
the behavior would have changed from gcc-12 to gcc-13.