[libc++] ADL-proof <iterator>. `__convert_to_integral` is not a customization point.

[• Quuxplusone]

Summary

The interesting change here is that we no longer consider __convert_to_integral an ADL customization point for the user's types. I think the new behavior is defensible. The old behavior had come from D7449, where Marshall explicitly said "people can't define their own [__convert_to_integral overloads]."

libcxx/include/algorithm

 #if _LIBCPP_STD_VER > 14
 // for_each_n
 
 template <class _InputIterator, class _Size, class _Function>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _InputIterator
 for_each_n(_InputIterator __first, _Size __orig_n, _Function __f)
 {
-    typedef decltype(__convert_to_integral(__orig_n)) _IntegralSize;
+    typedef decltype(_VSTD::__convert_to_integral(__orig_n)) _IntegralSize;
     _IntegralSize __n = __orig_n;
     while (__n > 0)
     {
          __f(*__first);
          ++__first;
          --__n;
     }
     return __first;

 template <class _ForwardIterator, class _Size, class _Tp, class _BinaryPredicate>
 _LIBCPP_NODISCARD_EXT inline
 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _ForwardIterator
 search_n(_ForwardIterator __first, _ForwardIterator __last,
          _Size __count, const _Tp& __value_, _BinaryPredicate __pred)
 {
     return _VSTD::__search_n<typename add_lvalue_reference<_BinaryPredicate>::type>
-           (__first, __last, __convert_to_integral(__count), __value_, __pred,
+           (__first, __last, _VSTD::__convert_to_integral(__count), __value_, __pred,
            typename iterator_traits<_ForwardIterator>::iterator_category());
 }
 
 template <class _ForwardIterator, class _Size, class _Tp>
 _LIBCPP_NODISCARD_EXT inline
 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _ForwardIterator
 search_n(_ForwardIterator __first, _ForwardIterator __last, _Size __count, const _Tp& __value_)
 {
     typedef typename iterator_traits<_ForwardIterator>::value_type __v;
-    return _VSTD::search_n(__first, __last, __convert_to_integral(__count),
+    return _VSTD::search_n(__first, __last, _VSTD::__convert_to_integral(__count),
                            __value_, __equal_to<__v, _Tp>());
 }
 
 // copy
 template <class _Iter>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 _Iter
 __unwrap_iter(_Iter __i)

 typename enable_if
 <
     __is_cpp17_input_iterator<_InputIterator>::value &&
    !__is_cpp17_random_access_iterator<_InputIterator>::value,
     _OutputIterator
 >::type
 copy_n(_InputIterator __first, _Size __orig_n, _OutputIterator __result)
 {
-    typedef decltype(__convert_to_integral(__orig_n)) _IntegralSize;
+    typedef decltype(_VSTD::__convert_to_integral(__orig_n)) _IntegralSize;
     _IntegralSize __n = __orig_n;
     if (__n > 0)
     {
         *__result = *__first;
         ++__result;
         for (--__n; __n > 0; --__n)
         {
             ++__first;
             *__result = *__first;
             ++__result;
         }
     }
     return __result;
 }
 
 template<class _InputIterator, class _Size, class _OutputIterator>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename enable_if
 <
     __is_cpp17_random_access_iterator<_InputIterator>::value,
     _OutputIterator
 >::type
 copy_n(_InputIterator __first, _Size __orig_n, _OutputIterator __result)
 {
-    typedef decltype(__convert_to_integral(__orig_n)) _IntegralSize;
+    typedef decltype(_VSTD::__convert_to_integral(__orig_n)) _IntegralSize;
     _IntegralSize __n = __orig_n;
     return _VSTD::copy(__first, __first + __n, __result);
 }
 
 // move
 
 // __move_constexpr exists so that __move doesn't call itself when delegating to the constexpr
 // version of __move.
__fill_n(_OutputIterator __first, _Size __n, const _Tp& __value_)
     return __first;
 }
 
 template <class _OutputIterator, class _Size, class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _OutputIterator
 fill_n(_OutputIterator __first, _Size __n, const _Tp& __value_)
 {
-   return _VSTD::__fill_n(__first, __convert_to_integral(__n), __value_);
+   return _VSTD::__fill_n(__first, _VSTD::__convert_to_integral(__n), __value_);
 }
 
 // fill
 
 template <class _ForwardIterator, class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 void
 __fill(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value_, forward_iterator_tag)

 
 // generate_n
 
 template <class _OutputIterator, class _Size, class _Generator>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _OutputIterator
 generate_n(_OutputIterator __first, _Size __orig_n, _Generator __gen)
 {
-    typedef decltype(__convert_to_integral(__orig_n)) _IntegralSize;
+    typedef decltype(_VSTD::__convert_to_integral(__orig_n)) _IntegralSize;
     _IntegralSize __n = __orig_n;
     for (; __n > 0; ++__first, (void) --__n)
         *__first = __gen();
     return __first;
 }
 
 // remove
 

libcxx/include/iterator

 }
 
 template <class _InputIter, class _Distance>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 void advance(_InputIter& __i, _Distance __orig_n)
 {
     _LIBCPP_ASSERT(__orig_n >= 0 || __is_cpp17_bidirectional_iterator<_InputIter>::value,
                    "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
-    typedef decltype(__convert_to_integral(__orig_n)) _IntegralSize;
+    typedef decltype(_VSTD::__convert_to_integral(__orig_n)) _IntegralSize;
     _IntegralSize __n = __orig_n;
-    __advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
+    _VSTD::__advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
 }
 
 template <class _InputIter>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 typename iterator_traits<_InputIter>::difference_type
 __distance(_InputIter __first, _InputIter __last, input_iterator_tag)
 {
     typename iterator_traits<_InputIter>::difference_type __r(0);
__distance(_RandIter __first, _RandIter __last, random_access_iterator_tag)
     return __last - __first;
 }
 
 template <class _InputIter>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 typename iterator_traits<_InputIter>::difference_type
 distance(_InputIter __first, _InputIter __last)
 {
-    return __distance(__first, __last, typename iterator_traits<_InputIter>::iterator_category());
+    return _VSTD::__distance(__first, __last, typename iterator_traits<_InputIter>::iterator_category());
 }
 
 template <class _InputIter>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 typename enable_if
 <
     __is_cpp17_input_iterator<_InputIter>::value,
     _InputIter

libcxx/test/std/iterators/iterator.primitives/iterator.operations/robust_against_adl.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <iterator>
+
+#include <iterator>
+
+#include "test_macros.h"
+
+struct Incomplete;
+template<class T> struct Holder { T t; };
+
+template<class>
+struct Intable {
+    operator int() const { return 1; }
+};
+
+int main() {
+    Holder<Incomplete> *a[2] = {};
+    Holder<Incomplete> **p = a;
+#if TEST_STD_VER >= 17
+    p = std::next(p);
+    p = std::prev(p);
+    p = std::next(p, 2);
+    p = std::prev(p, 2);
+#endif
+    std::advance(p, Intable<Holder<Incomplete> >());
+    (void)std::distance(p, p);
+}