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From: "whatwasthataddress at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/96416] New: address_of() is broken by static_assert
 in pointer_traits
Date: Sun, 02 Aug 2020 04:17:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96416

            Bug ID: 96416
           Summary: address_of() is broken by static_assert in
                    pointer_traits
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: whatwasthataddress at gmail dot com
  Target Milestone: ---

pointer_traits contains this static_assert:

      static_assert(!is_same<element_type, __undefined>::value,
          "pointer type defines element_type or is like SomePointer<T, Args=
>");

When trying to use address_of() with an iterator that has a usable
operator->(), address_of() fails because the static_assert causes a hard er=
ror
in the trailing decltype here:

  template<typename _Ptr>
    constexpr auto
    __to_address(const _Ptr& __ptr) noexcept
    -> decltype(std::pointer_traits<_Ptr>::to_address(__ptr))
    { return std::pointer_traits<_Ptr>::to_address(__ptr); }

  template<typename _Ptr, typename... _None>
    constexpr auto
    __to_address(const _Ptr& __ptr, _None...) noexcept
    {
      if constexpr (is_base_of_v<__gnu_debug::_Safe_iterator_base, _Ptr>)
        return std::__to_address(__ptr.base().operator->());
      else
        return std::__to_address(__ptr.operator->());
    }

When I comment out the static_assert, to_address() works with the iterator =
in
question.=