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[libc++] Disallow std::prev(non_cpp17_bidi_iterator) #112102

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Merged
merged 4 commits into from
Oct 23, 2024
Merged

[libc++] Disallow std::prev(non_cpp17_bidi_iterator) #112102

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[libc++] Disallow std::prev(non_cpp17_bidi_iterator)
huixie90 Oct 12, 2024
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huixie90 Oct 13, 2024
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address code review
huixie90 Oct 19, 2024
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huixie90 Oct 19, 2024
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15 changes: 14 additions & 1 deletion libcxx/include/__iterator/prev.h
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Original file line number Diff line number Diff line change
Expand Up @@ -17,16 +17,20 @@
#include <__iterator/incrementable_traits.h>
#include <__iterator/iterator_traits.h>
#include <__type_traits/enable_if.h>
#include <__utility/move.h>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

_LIBCPP_BEGIN_NAMESPACE_STD

template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter
prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n = 1) {
prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n) {
// Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
// Note that this check duplicates the similar check in `std::advance`.
_LIBCPP_ASSERT_PEDANTIC(__n <= 0 || __has_bidirectional_iterator_category<_InputIter>::value,
Expand All @@ -35,6 +39,13 @@ prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n =
return __x;
}

template <class _BidirectionalIterator,
__enable_if_t<__has_bidirectional_iterator_category<_BidirectionalIterator>::value, int> = 0>
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _BidirectionalIterator
prev(_BidirectionalIterator __it) {
return std::prev(std::move(__it), 1);
}

#if _LIBCPP_STD_VER >= 20

// [range.iter.op.prev]
Expand Down Expand Up @@ -70,4 +81,6 @@ inline constexpr auto prev = __prev{};

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#endif // _LIBCPP___ITERATOR_PREV_H
15 changes: 15 additions & 0 deletions libcxx/test/std/iterators/iterator.primitives/iterator.operations/prev.pass.cpp
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Expand Up @@ -18,6 +18,21 @@
#include "test_macros.h"
#include "test_iterators.h"

template <class Iter>
std::false_type prev_test(...);

template <class Iter>
decltype((void)std::prev(std::declval<Iter>()), std::true_type()) prev_test(int);

template <class Iter>
using CanPrev = decltype(prev_test<Iter>(0));

static_assert(!CanPrev<cpp17_input_iterator<int*> >::value, "");
static_assert(CanPrev<bidirectional_iterator<int*> >::value, "");
#if TEST_STD_VER >= 20
static_assert(!CanPrev<cpp20_random_access_iterator<int*> >::value);
#endif

template <class It>
TEST_CONSTEXPR_CXX17 void
check_prev_n(It it, typename std::iterator_traits<It>::difference_type n, It result)
Expand Down
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