[libc++] std::prev(it) should not compile for a non-bidi iterator #109456
Description
std::prev(it, n) could conceivably be well-formed for a non-bidirectional it if n is negative, however std::prev(it) is always UB, and we can easily diagnose it at compile-time. Right now std::prev(non_bidi) is a no-op, which is absolutely vexing.
https://godbolt.org/z/zaG3jTEqP
Potential patch:
diff --git a/libcxx/include/__iterator/prev.h b/libcxx/include/__iterator/prev.h index e950d8dc4147..98883188312a 100644 --- a/libcxx/include/__iterator/prev.h +++ b/libcxx/include/__iterator/prev.h @@ -26,7 +26,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0> 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 +35,12 @@ 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> +_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]