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

[huixie90]

The std::prev function appeared to work on non Cpp17BidirectionalIterators, however it behaved strangely by being the identity function. That was extremely misleading and potentially dangerous, since several recent iterators that are C++20 bidirectional_iterators don't satisfy the Cpp17BidirectionalIterator named requirement. For example:

auto zv = std::views::zip(vec1, vec2);
auto it = zv.begin() + 5;
auto it2 = std::prev(it); // "it2" will be the same as "it",  instead of the previous one

Here, zip_view::iterator is a c++20 random_access_iterator, but it only satisfies the Cpp17InputIterator named requirement because its reference type is a proxy type. Hence std::prev would silently accept that iterator but it would do a no-op instead of going to the previous position.

This patch changes std::prev to produce an error at compile-time when instantiated with a type that is not a Cpp17BidirectionalIterator.

Fixes #109456

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Hui (huixie90)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/112102.diff

2 Files Affected:

• (modified) libcxx/include/__iterator/prev.h (+9-1)
• (modified) libcxx/test/std/iterators/iterator.primitives/iterator.operations/prev.pass.cpp (+15)

diff --git a/libcxx/include/__iterator/prev.h b/libcxx/include/__iterator/prev.h
index 7e97203836eb98..47bfb089504818 100644
--- a/libcxx/include/__iterator/prev.h
+++ b/libcxx/include/__iterator/prev.h
@@ -17,6 +17,7 @@
 #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
@@ -26,7 +27,7 @@ _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,
@@ -35,6 +36,13 @@ 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]
diff --git a/libcxx/test/std/iterators/iterator.primitives/iterator.operations/prev.pass.cpp b/libcxx/test/std/iterators/iterator.primitives/iterator.operations/prev.pass.cpp
index 784c1b93b7ca89..be0295d4f7c165 100644
--- a/libcxx/test/std/iterators/iterator.primitives/iterator.operations/prev.pass.cpp
+++ b/libcxx/test/std/iterators/iterator.primitives/iterator.operations/prev.pass.cpp
@@ -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)

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.