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From: "hewillk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/106376] New: The implementation of
 std::pointer_traits seems wrong
Date: Thu, 21 Jul 2022 03:56:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106376

            Bug ID: 106376
           Summary: The implementation of std::pointer_traits seems wrong
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

[pointer.traits.types]:=20
using element_type =3D see below;
Type: Ptr=E2=80=8B::=E2=80=8Belement_=C2=ADtype if the qualified-id Ptr=E2=
=80=8B::=E2=80=8Belement_=C2=ADtype is valid
and denotes a type ([temp.deduct]); otherwise, T if Ptr is a class template
instantiation of the form SomePointer<T, Args>, where Args is zero or more =
type
arguments; otherwise, the specialization is *ill-formed*.

So for the following, the instantiation of pointer_traits will be ill-forme=
d,
and static_assert will cause a hard error, just like libc++ and MSVC-STL do.
libstdc++ specifies element_type as a meaningless type, so the static_assert
still   passes.

Although the behavior of libstdc++ is incorrect, but I am not feel good abo=
ut
this static_assert hard error..

#include <iterator>

struct I {
  using iterator_category =3D std::contiguous_iterator_tag;
  using difference_type =3D std::ptrdiff_t;
  using value_type =3D int;
  value_type& operator*() const;
  I& operator++();
  I operator++(int);
  I& operator--();
  I operator--(int);
  I& operator+=3D(difference_type);
  I& operator-=3D(difference_type);
  value_type* operator->() const;
  value_type& operator[](difference_type) const;
  friend I operator+(I, difference_type);
  friend I operator+(difference_type, I);
  friend I operator-(I, difference_type);
  friend difference_type operator-(I, I);
  auto operator<=3D>(const I&) const =3D default;
};

static_assert(std::contiguous_iterator<I>);

https://godbolt.org/z/sx763oMn6=