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Implement LWG-4218 Constraint recursion in basic_const_iterator's relational operators due to ADL + CWG-2369 #6324

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Implement LWG-4218 Constraint recursion in basic_const_iterator's relational operators due to ADL + CWG-2369 #6324

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1374f01
Implement LWG-4218
frederick-vs-ja Jun 23, 2026
9cfc309
Workaround for DevCom-11111696
frederick-vs-ja Jun 23, 2026
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36 changes: 28 additions & 8 deletions stl/inc/xutility
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Original file line number Diff line number Diff line change
Expand Up @@ -2605,30 +2605,50 @@ public:
return _Current <=> _Right;
}

template <_Not_a_const_iterator _Other>
template <_Not_a_const_iterator _Other, same_as<_Iter> _Jter
#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, DevCom-11111696
,
_Iter* = nullptr
#endif // ^^^ workaround ^^^
>
requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Other>
_NODISCARD friend constexpr bool operator<(const _Other& _Left, const basic_const_iterator& _Right)
_NODISCARD friend constexpr bool operator<(const _Other& _Left, const basic_const_iterator<_Jter>& _Right)
noexcept(noexcept(_STD _Fake_copy_init<bool>(_Left < _Right._Current))) /* strengthened */ {
return _Left < _Right._Current;
}

template <_Not_a_const_iterator _Other>
template <_Not_a_const_iterator _Other, same_as<_Iter> _Jter
#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, DevCom-11111696
,
_Iter* = nullptr
#endif // ^^^ workaround ^^^
>
requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Other>
_NODISCARD friend constexpr bool operator>(const _Other& _Left, const basic_const_iterator& _Right)
_NODISCARD friend constexpr bool operator>(const _Other& _Left, const basic_const_iterator<_Jter>& _Right)
noexcept(noexcept(_STD _Fake_copy_init<bool>(_Left > _Right._Current))) /* strengthened */ {
return _Left > _Right._Current;
}

template <_Not_a_const_iterator _Other>
template <_Not_a_const_iterator _Other, same_as<_Iter> _Jter
#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, DevCom-11111696
,
_Iter* = nullptr
#endif // ^^^ workaround ^^^
>
requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Other>
_NODISCARD friend constexpr bool operator<=(const _Other& _Left, const basic_const_iterator& _Right)
_NODISCARD friend constexpr bool operator<=(const _Other& _Left, const basic_const_iterator<_Jter>& _Right)
noexcept(noexcept(_STD _Fake_copy_init<bool>(_Left <= _Right._Current))) /* strengthened */ {
return _Left <= _Right._Current;
}

template <_Not_a_const_iterator _Other>
template <_Not_a_const_iterator _Other, same_as<_Iter> _Jter
#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, DevCom-11111696
,
_Iter* = nullptr
#endif // ^^^ workaround ^^^
>
requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Other>
_NODISCARD friend constexpr bool operator>=(const _Other& _Left, const basic_const_iterator& _Right)
_NODISCARD friend constexpr bool operator>=(const _Other& _Left, const basic_const_iterator<_Jter>& _Right)
noexcept(noexcept(_STD _Fake_copy_init<bool>(_Left >= _Right._Current))) /* strengthened */ {
return _Left >= _Right._Current;
}
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26 changes: 26 additions & 0 deletions tests/std/tests/P2278R4_basic_const_iterator/test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -270,6 +270,32 @@ static constexpr int some_ints[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
using Zipped = decltype(views::zip(some_ints) | views::as_const | views::as_rvalue);
static_assert(same_as<ranges::range_reference_t<Zipped>, tuple<const int&&>>);

// LWG-4218 "Constraint recursion in basic_const_iterator's relational operators due to ADL + CWG 2369"
namespace lwg_4218 {
template <class T>
struct adl_conversion_source {
operator T() const;

friend bool operator==(const adl_conversion_source&, const adl_conversion_source&) {
return true;
}
friend bool operator==(const adl_conversion_source&, const T&) {
return true;
}

template <same_as<adl_conversion_source> Self>
friend partial_ordering operator<=>(const adl_conversion_source&, const Self&) {
return partial_ordering::equivalent;
}
};

using cvi = basic_const_iterator<vector<int>::iterator>;
static_assert(totally_ordered_with<adl_conversion_source<cvi>, cvi>);

using rcvi = reverse_iterator<cvi>;
static_assert(totally_ordered<rcvi>);
} // namespace lwg_4218

struct instantiator {
template <input_iterator It>
static constexpr void call() {
Expand Down