[libc++] Remove redundant and somewhat confusing assertions around advance() (#133276)

The std::advance function has a clear precondition that it can only be
called with a negative distance when a bidirectional iterator is used.
However, prev() and next() don't have such preconditions explicitly,
they inherit it from calling advance().

This patch removes assertions in prev() and next() that were duplicates
of similar ones in advance(), and removes a copy-pasted comment that was
trying to justify the use of _LIBCPP_ASSERT_PEDANTIC but IMO is creating
confusion with little benefit.

Author: ldionne

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;
 }