[libcxx-commits] [libcxx] [libc++][C++03] cherry-pick #112102 (PR #157104)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 created https://github.com/llvm/llvm-project/pull/157104

None

>From 76b37d5b08d3c374d3deb74eaa757e1a736a7d11 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Fri, 5 Sep 2025 15:19:33 +0200
Subject: [PATCH] [libc++][C++03] cherry-pick #112102

---
 libcxx/include/__cxx03/__iterator/prev.h       | 18 +++++++++++++++++-
 .../iterator.operations/prev.verify.cpp        |  2 --
 2 files changed, 17 insertions(+), 3 deletions(-)

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"