[libc++] Assume that __wrap_iter always wraps a fancy pointer.

Not only do we conscientiously avoid using `__wrap_iter` for non-contiguous
iterators (in vector, string, span...) but also we make the assumption
(in regex) that `__wrap_iter<_Iter>` is contiguous for all `_Iter`.

So `__wrap_iter<reverse_iterator<int*>>` should be considered IFNDR,
and every `__wrap_iter` should correctly advertise contiguity in C++20.

Drive-by simplify some type traits.

Reviewed as part of https://reviews.llvm.org/D102781

1 files changed, 8 insertions, 6 deletions

diff --git a/libcxx/include/__iterator/iterator_traits.h b/libcxx/include/__iterator/iterator_traits.h
index ebffc974f0e1..307ee143c1d2 100644
--- a/libcxx/include/__iterator/iterator_traits.h
+++ b/libcxx/include/__iterator/iterator_traits.h
@@ -428,7 +428,7 @@ struct _LIBCPP_TEMPLATE_VIS iterator_traits<_Tp*>
 
 template <class _Tp, class _Up, bool = __has_iterator_category<iterator_traits<_Tp> >::value>
 struct __has_iterator_category_convertible_to
-    : _BoolConstant<is_convertible<typename iterator_traits<_Tp>::iterator_category, _Up>::value>
+    : is_convertible<typename iterator_traits<_Tp>::iterator_category, _Up>
 {};
 
 template <class _Tp, class _Up>
@@ -436,7 +436,7 @@ struct __has_iterator_category_convertible_to<_Tp, _Up, false> : false_type {};
 
 template <class _Tp, class _Up, bool = __has_iterator_concept<_Tp>::value>
 struct __has_iterator_concept_convertible_to
-    : _BoolConstant<is_convertible<typename _Tp::iterator_concept, _Up>::value>
+    : is_convertible<typename _Tp::iterator_concept, _Up>
 {};
 
 template <class _Tp, class _Up>
@@ -454,10 +454,12 @@ struct __is_cpp17_bidirectional_iterator : public __has_iterator_category_conver
 template <class _Tp>
 struct __is_cpp17_random_access_iterator : public __has_iterator_category_convertible_to<_Tp, random_access_iterator_tag> {};
 
-// __is_cpp17_contiguous_iterator determines if an iterator is contiguous,
-// either because it advertises itself as such (in C++20) or because it
-// is a pointer type or a known trivial wrapper around a pointer type,
-// such as __wrap_iter<T*>.
+// __is_cpp17_contiguous_iterator determines if an iterator is known by
+// libc++ to be contiguous, either because it advertises itself as such
+// (in C++20) or because it is a pointer type or a known trivial wrapper
+// around a (possibly fancy) pointer type, such as __wrap_iter<T*>.
+// Such iterators receive special "contiguous" optimizations in
+// std::copy and std::sort.
 //
 #if _LIBCPP_STD_VER > 17
 template <class _Tp>