[libcxx-commits] [libcxx] [libc++][C++03] cherry-pick #112102 (PR #157104)
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Fri Sep 5 06:29:16 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: Nikolas Klauser (philnik777)
<details>
<summary>Changes</summary>
---
Full diff: https://github.com/llvm/llvm-project/pull/157104.diff
2 Files Affected:
- (modified) libcxx/include/__cxx03/__iterator/prev.h (+17-1)
- (modified) libcxx/test/libcxx-03/iterators/iterator.primitives/iterator.operations/prev.verify.cpp (-2)
``````````diff
diff --git a/libcxx/include/__cxx03/__iterator/prev.h b/libcxx/include/__cxx03/__iterator/prev.h
index dc1bdaf584022..32f6e66dac9a5 100644
--- a/libcxx/include/__cxx03/__iterator/prev.h
+++ b/libcxx/include/__cxx03/__iterator/prev.h
@@ -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,
@@ -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
diff --git a/libcxx/test/libcxx-03/iterators/iterator.primitives/iterator.operations/prev.verify.cpp b/libcxx/test/libcxx-03/iterators/iterator.primitives/iterator.operations/prev.verify.cpp
index ffef687723983..da0a336815a5c 100644
--- a/libcxx/test/libcxx-03/iterators/iterator.primitives/iterator.operations/prev.verify.cpp
+++ b/libcxx/test/libcxx-03/iterators/iterator.primitives/iterator.operations/prev.verify.cpp
@@ -8,8 +8,6 @@
// std::prev
-// XFAIL: FROZEN-CXX03-HEADERS-FIXME
-
#include <iterator>
#include "test_iterators.h"
``````````
</details>
https://github.com/llvm/llvm-project/pull/157104
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