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From: "ldalessandro at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/109751] New: boost interface_interface fails concept
 check starting in gcc-13
Date: Fri, 05 May 2023 17:45:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109751

            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_i=
terator.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>,=
=20
  std::forward_iterator_tag, T>
  {
    using base_type =3D boost::stl_interfaces::iterator_interface<iterator<=
T>,
std::forward_iterator_tag, T>;
    using base_type::operator++;

    T& operator*() const;
    iterator& operator++();
    bool operator=3D=3D(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 w=
hy
the behavior would have changed from gcc-12 to gcc-13.=