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[libc++][C++03] cherry-pick #112102 #157104

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Merged
merged 1 commit into from
Sep 9, 2025
+17 −3
Merged

[libc++][C++03] cherry-pick #112102 #157104

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18 changes: 17 additions & 1 deletion libcxx/include/__cxx03/__iterator/prev.h
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Original file line number Diff line number Diff line change
Expand Up @@ -15,16 +15,20 @@
#include <__cxx03/__iterator/advance.h>
#include <__cxx03/__iterator/iterator_traits.h>
#include <__cxx03/__type_traits/enable_if.h>
#include <__cxx03/__utility/move.h>

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

_LIBCPP_PUSH_MACROS
#include <__cxx03/__undef_macros>

_LIBCPP_BEGIN_NAMESPACE_STD

template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
inline _LIBCPP_HIDE_FROM_ABI _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 @@ -33,6 +37,18 @@ prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n =
return __x;
}

// LWG 3197
// It is unclear what the implications of "BidirectionalIterator" in the standard are.
// However, calling std::prev(non-bidi-iterator) is obviously an error and we should catch it at compile time.
template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _InputIter prev(_InputIter __it) {
static_assert(__has_bidirectional_iterator_category<_InputIter>::value,
"Attempt to prev(it) with a non-bidirectional iterator");
return std::prev(std::move(__it), 1);
}

_LIBCPP_POP_MACROS

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___CXX03___ITERATOR_PREV_H
2 changes: 0 additions & 2 deletions libcxx/test/libcxx-03/iterators/iterator.primitives/iterator.operations/prev.verify.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -8,8 +8,6 @@

// std::prev

// XFAIL: FROZEN-CXX03-HEADERS-FIXME

#include <iterator>
#include "test_iterators.h"

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