[libc++] Remove redundant and somewhat confusing assertions around advance()

libcxx/include/__iterator/advance.h

@@ -65,8 +65,7 @@ template < class _InputIter,
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 void advance(_InputIter& __i, _Distance __orig_n) {
   typedef typename iterator_traits<_InputIter>::difference_type _Difference;
   _Difference __n = static_cast<_Difference>(std::__convert_to_integral(__orig_n));
-  // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
-  _LIBCPP_ASSERT_PEDANTIC(__n >= 0 || __has_bidirectional_iterator_category<_InputIter>::value,
+  _LIBCPP_ASSERT_PEDANTIC(__has_bidirectional_iterator_category<_InputIter>::value || __n >= 0,
                           "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
   std::__advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
 }
@@ -98,9 +97,8 @@ struct __advance {
   // Preconditions: If `I` does not model `bidirectional_iterator`, `n` is not negative.
   template <input_or_output_iterator _Ip>
   _LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, iter_difference_t<_Ip> __n) const {
-    // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
-    _LIBCPP_ASSERT_PEDANTIC(
-        __n >= 0 || bidirectional_iterator<_Ip>, "If `n < 0`, then `bidirectional_iterator<I>` must be true.");
+    _LIBCPP_ASSERT_PEDANTIC(bidirectional_iterator<_Ip> || __n >= 0,
+                            "ranges::advance: Can only pass a negative `n` with a bidirectional_iterator.");
 
     // If `I` models `random_access_iterator`, equivalent to `i += n`.
     if constexpr (random_access_iterator<_Ip>) {
@@ -149,9 +147,9 @@ struct __advance {
   template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
   _LIBCPP_HIDE_FROM_ABI constexpr iter_difference_t<_Ip>
   operator()(_Ip& __i, iter_difference_t<_Ip> __n, _Sp __bound_sentinel) const {
-    // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
-    _LIBCPP_ASSERT_PEDANTIC((__n >= 0) || (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>),
-                            "If `n < 0`, then `bidirectional_iterator<I> && same_as<I, S>` must be true.");
+    _LIBCPP_ASSERT_PEDANTIC(
+        (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>) || (__n >= 0),
+        "ranges::advance: Can only pass a negative `n` with a bidirectional_iterator coming from a common_range.");
     // If `S` and `I` model `sized_sentinel_for<S, I>`:
     if constexpr (sized_sentinel_for<_Sp, _Ip>) {
       // If |n| >= |bound_sentinel - i|, equivalent to `ranges::advance(i, bound_sentinel)`.

libcxx/include/__iterator/next.h

@@ -10,7 +10,6 @@
 #ifndef _LIBCPP___ITERATOR_NEXT_H
 #define _LIBCPP___ITERATOR_NEXT_H
 
-#include <__assert>
 #include <__config>
 #include <__iterator/advance.h>
 #include <__iterator/concepts.h>
@@ -27,11 +26,6 @@ _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
 next(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n = 1) {
-  // 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,
-                          "Attempt to next(it, n) with negative n on a non-bidirectional iterator");
-
   std::advance(__x, __n);
   return __x;
 }

libcxx/include/__iterator/prev.h

@@ -10,7 +10,6 @@
 #ifndef _LIBCPP___ITERATOR_PREV_H
 #define _LIBCPP___ITERATOR_PREV_H
 
-#include <__assert>
 #include <__config>
 #include <__iterator/advance.h>
 #include <__iterator/concepts.h>
@@ -31,10 +30,6 @@ _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) {
-  // 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,
-                          "Attempt to prev(it, n) with a positive n on a non-bidirectional iterator");
   std::advance(__x, -__n);
   return __x;
 }

libcxx/test/libcxx/iterators/assert.next.pass.cpp

@@ -25,7 +25,7 @@ int main(int, char**) {
     forward_iterator<int *> it(a+1);
     (void)std::next(it, 1); // should work fine
     (void)std::next(it, 0); // should work fine
-    TEST_LIBCPP_ASSERT_FAILURE(std::next(it, -1), "Attempt to next(it, n) with negative n on a non-bidirectional iterator");
+    TEST_LIBCPP_ASSERT_FAILURE(std::next(it, -1), "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
 
     return 0;
 }

libcxx/test/libcxx/iterators/assert.prev.pass.cpp

@@ -31,7 +31,7 @@ int main(int, char**) {
     forward_iterator<int *> it(a+1);
     (void)std::prev(it, -1); // should work fine
     (void)std::prev(it, 0);  // should work fine
-    TEST_LIBCPP_ASSERT_FAILURE(std::prev(it, 1), "Attempt to prev(it, n) with a positive n on a non-bidirectional iterator");
+    TEST_LIBCPP_ASSERT_FAILURE(std::prev(it, 1), "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
 
     return 0;
 }