diff --git a/libcxx/include/__algorithm/copy_n.h b/libcxx/include/__algorithm/copy_n.h
--- a/libcxx/include/__algorithm/copy_n.h
+++ b/libcxx/include/__algorithm/copy_n.h
@@ -22,10 +22,7 @@
 
 template<class _InputIterator, class _Size, class _OutputIterator>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-typename enable_if
-<
-    __is_cpp17_input_iterator<_InputIterator>::value &&
-   !__is_cpp17_random_access_iterator<_InputIterator>::value,
+typename enable_if <__is_exactly_cpp17_input_iterator<_InputIterator>::value,
     _OutputIterator
 >::type
 copy_n(_InputIterator __first, _Size __orig_n, _OutputIterator __result)
diff --git a/libcxx/include/__iterator/iterator_traits.h b/libcxx/include/__iterator/iterator_traits.h
--- a/libcxx/include/__iterator/iterator_traits.h
+++ b/libcxx/include/__iterator/iterator_traits.h
@@ -475,6 +475,12 @@
          __has_iterator_category_convertible_to<_Tp, input_iterator_tag>::value &&
         !__has_iterator_category_convertible_to<_Tp, forward_iterator_tag>::value> {};
 
+template <class _Tp>
+struct __is_exactly_cpp17_forward_iterator
+    : public integral_constant<bool,
+         __has_iterator_category_convertible_to<_Tp, forward_iterator_tag>::value &&
+        !__has_iterator_category_convertible_to<_Tp, bidirectional_iterator_tag>::value> {};
+
 template<class _InputIterator>
 using __iter_value_type = typename iterator_traits<_InputIterator>::value_type;
 
diff --git a/libcxx/include/__split_buffer b/libcxx/include/__split_buffer
--- a/libcxx/include/__split_buffer
+++ b/libcxx/include/__split_buffer
@@ -118,7 +118,7 @@
     void __construct_at_end(size_type __n);
     void __construct_at_end(size_type __n, const_reference __x);
     template <class _InputIter>
-    __enable_if_t<__is_cpp17_input_iterator<_InputIter>::value && !__is_cpp17_forward_iterator<_InputIter>::value>
+    __enable_if_t<__is_exactly_cpp17_input_iterator<_InputIter>::value>
         __construct_at_end(_InputIter __first, _InputIter __last);
     template <class _ForwardIterator>
     __enable_if_t<__is_cpp17_forward_iterator<_ForwardIterator>::value>
diff --git a/libcxx/include/deque b/libcxx/include/deque
--- a/libcxx/include/deque
+++ b/libcxx/include/deque
@@ -1354,8 +1354,7 @@
 
     template <class _InputIter>
         void assign(_InputIter __f, _InputIter __l,
-                    typename enable_if<__is_cpp17_input_iterator<_InputIter>::value &&
-                                      !__is_cpp17_random_access_iterator<_InputIter>::value>::type* = 0);
+                    typename enable_if<__is_exactly_cpp17_input_iterator<_InputIter>::value>::type* = 0);
     template <class _RAIter>
         void assign(_RAIter __f, _RAIter __l,
                     typename enable_if<__is_cpp17_random_access_iterator<_RAIter>::value>::type* = 0);
@@ -1458,12 +1457,10 @@
     iterator insert(const_iterator __p, size_type __n, const value_type& __v);
     template <class _InputIter>
         iterator insert(const_iterator __p, _InputIter __f, _InputIter __l,
-                         typename enable_if<__is_cpp17_input_iterator<_InputIter>::value
-                                         &&!__is_cpp17_forward_iterator<_InputIter>::value>::type* = 0);
+                         typename enable_if<__is_exactly_cpp17_input_iterator<_InputIter>::value>::type* = 0);
     template <class _ForwardIterator>
         iterator insert(const_iterator __p, _ForwardIterator __f, _ForwardIterator __l,
-                               typename enable_if<__is_cpp17_forward_iterator<_ForwardIterator>::value
-                                         &&!__is_cpp17_bidirectional_iterator<_ForwardIterator>::value>::type* = 0);
+                        typename enable_if<__is_exactly_cpp17_forward_iterator<_ForwardIterator>::value>::type* = 0);
     template <class _BiIter>
         iterator insert(const_iterator __p, _BiIter __f, _BiIter __l,
                          typename enable_if<__is_cpp17_bidirectional_iterator<_BiIter>::value>::type* = 0);
@@ -1550,8 +1547,7 @@
 
     template <class _InpIter>
         void __append(_InpIter __f, _InpIter __l,
-                 typename enable_if<__is_cpp17_input_iterator<_InpIter>::value &&
-                                   !__is_cpp17_forward_iterator<_InpIter>::value>::type* = 0);
+                 typename enable_if<__is_exactly_cpp17_input_iterator<_InpIter>::value>::type* = 0);
     template <class _ForIter>
         void __append(_ForIter __f, _ForIter __l,
                       typename enable_if<__is_cpp17_forward_iterator<_ForIter>::value>::type* = 0);
@@ -1758,8 +1754,7 @@
 template <class _InputIter>
 void
 deque<_Tp, _Allocator>::assign(_InputIter __f, _InputIter __l,
-                               typename enable_if<__is_cpp17_input_iterator<_InputIter>::value &&
-                                                 !__is_cpp17_random_access_iterator<_InputIter>::value>::type*)
+                               typename enable_if<__is_exactly_cpp17_input_iterator<_InputIter>::value>::type*)
 {
     iterator __i = __base::begin();
     iterator __e = __base::end();
@@ -2260,8 +2255,7 @@
 template <class _InputIter>
 typename deque<_Tp, _Allocator>::iterator
 deque<_Tp, _Allocator>::insert(const_iterator __p, _InputIter __f, _InputIter __l,
-                               typename enable_if<__is_cpp17_input_iterator<_InputIter>::value
-                                               &&!__is_cpp17_forward_iterator<_InputIter>::value>::type*)
+                               typename enable_if<__is_exactly_cpp17_input_iterator<_InputIter>::value>::type*)
 {
     __split_buffer<value_type, allocator_type&> __buf(__base::__alloc());
     __buf.__construct_at_end(__f, __l);
@@ -2273,8 +2267,7 @@
 template <class _ForwardIterator>
 typename deque<_Tp, _Allocator>::iterator
 deque<_Tp, _Allocator>::insert(const_iterator __p, _ForwardIterator __f, _ForwardIterator __l,
-                               typename enable_if<__is_cpp17_forward_iterator<_ForwardIterator>::value
-                                               &&!__is_cpp17_bidirectional_iterator<_ForwardIterator>::value>::type*)
+                               typename enable_if<__is_exactly_cpp17_forward_iterator<_ForwardIterator>::value>::type*)
 {
     size_type __n = _VSTD::distance(__f, __l);
     __split_buffer<value_type, allocator_type&> __buf(__n, 0, __base::__alloc());
@@ -2356,8 +2349,7 @@
 template <class _InpIter>
 void
 deque<_Tp, _Allocator>::__append(_InpIter __f, _InpIter __l,
-                                 typename enable_if<__is_cpp17_input_iterator<_InpIter>::value &&
-                                                   !__is_cpp17_forward_iterator<_InpIter>::value>::type*)
+                                 typename enable_if<__is_exactly_cpp17_input_iterator<_InpIter>::value>::type*)
 {
     for (; __f != __l; ++__f)
 #ifdef _LIBCPP_CXX03_LANG
diff --git a/libcxx/include/regex b/libcxx/include/regex
--- a/libcxx/include/regex
+++ b/libcxx/include/regex
@@ -2743,12 +2743,7 @@
 
     template <class _InputIterator>
         _LIBCPP_INLINE_VISIBILITY
-        typename enable_if
-        <
-             __is_cpp17_input_iterator  <_InputIterator>::value &&
-            !__is_cpp17_forward_iterator<_InputIterator>::value,
-            basic_regex&
-        >::type
+        typename enable_if<__is_exactly_cpp17_input_iterator<_InputIterator>::value, basic_regex&>::type
         assign(_InputIterator __first, _InputIterator __last,
                             flag_type __f = regex_constants::ECMAScript)
         {
diff --git a/libcxx/include/vector b/libcxx/include/vector
--- a/libcxx/include/vector
+++ b/libcxx/include/vector
@@ -387,16 +387,14 @@
 
     template <class _InputIterator>
         vector(_InputIterator __first,
-               typename enable_if<__is_cpp17_input_iterator  <_InputIterator>::value &&
-                                 !__is_cpp17_forward_iterator<_InputIterator>::value &&
+               typename enable_if<__is_exactly_cpp17_input_iterator<_InputIterator>::value &&
                                  is_constructible<
                                     value_type,
                                     typename iterator_traits<_InputIterator>::reference>::value,
                                  _InputIterator>::type __last);
     template <class _InputIterator>
         vector(_InputIterator __first, _InputIterator __last, const allocator_type& __a,
-               typename enable_if<__is_cpp17_input_iterator  <_InputIterator>::value &&
-                                 !__is_cpp17_forward_iterator<_InputIterator>::value &&
+               typename enable_if<__is_exactly_cpp17_input_iterator<_InputIterator>::value &&
                                  is_constructible<
                                     value_type,
                                     typename iterator_traits<_InputIterator>::reference>::value>::type* = 0);
@@ -459,10 +457,7 @@
         _NOEXCEPT_((__noexcept_move_assign_container<_Allocator, __alloc_traits>::value));
 
     template <class _InputIterator>
-        typename enable_if
-        <
-             __is_cpp17_input_iterator  <_InputIterator>::value &&
-            !__is_cpp17_forward_iterator<_InputIterator>::value &&
+        typename enable_if <__is_exactly_cpp17_input_iterator<_InputIterator>::value &&
             is_constructible<
                  value_type,
                  typename iterator_traits<_InputIterator>::reference>::value,
@@ -592,10 +587,7 @@
 
     iterator insert(const_iterator __position, size_type __n, const_reference __x);
     template <class _InputIterator>
-        typename enable_if
-        <
-             __is_cpp17_input_iterator  <_InputIterator>::value &&
-            !__is_cpp17_forward_iterator<_InputIterator>::value &&
+        typename enable_if <__is_exactly_cpp17_input_iterator<_InputIterator>::value &&
             is_constructible<
                  value_type,
                  typename iterator_traits<_InputIterator>::reference>::value,
@@ -1084,8 +1076,7 @@
 template <class _Tp, class _Allocator>
 template <class _InputIterator>
 vector<_Tp, _Allocator>::vector(_InputIterator __first,
-       typename enable_if<__is_cpp17_input_iterator  <_InputIterator>::value &&
-                         !__is_cpp17_forward_iterator<_InputIterator>::value &&
+       typename enable_if<__is_exactly_cpp17_input_iterator<_InputIterator>::value &&
                          is_constructible<
                             value_type,
                             typename iterator_traits<_InputIterator>::reference>::value,
@@ -1099,8 +1090,7 @@
 template <class _Tp, class _Allocator>
 template <class _InputIterator>
 vector<_Tp, _Allocator>::vector(_InputIterator __first, _InputIterator __last, const allocator_type& __a,
-       typename enable_if<__is_cpp17_input_iterator  <_InputIterator>::value &&
-                         !__is_cpp17_forward_iterator<_InputIterator>::value &&
+       typename enable_if<__is_exactly_cpp17_input_iterator<_InputIterator>::value &&
                          is_constructible<
                             value_type,
                             typename iterator_traits<_InputIterator>::reference>::value>::type*)
@@ -1295,10 +1285,7 @@
 
 template <class _Tp, class _Allocator>
 template <class _InputIterator>
-typename enable_if
-<
-     __is_cpp17_input_iterator  <_InputIterator>::value &&
-    !__is_cpp17_forward_iterator<_InputIterator>::value &&
+typename enable_if <__is_exactly_cpp17_input_iterator<_InputIterator>::value &&
     is_constructible<
        _Tp,
        typename iterator_traits<_InputIterator>::reference>::value,
@@ -1745,10 +1732,7 @@
 
 template <class _Tp, class _Allocator>
 template <class _InputIterator>
-typename enable_if
-<
-     __is_cpp17_input_iterator  <_InputIterator>::value &&
-    !__is_cpp17_forward_iterator<_InputIterator>::value &&
+typename enable_if <__is_exactly_cpp17_input_iterator<_InputIterator>::value &&
     is_constructible<
        _Tp,
        typename iterator_traits<_InputIterator>::reference>::value,
@@ -2051,12 +2035,10 @@
     vector(size_type __n, const value_type& __v, const allocator_type& __a);
     template <class _InputIterator>
         vector(_InputIterator __first, _InputIterator __last,
-               typename enable_if<__is_cpp17_input_iterator  <_InputIterator>::value &&
-                                 !__is_cpp17_forward_iterator<_InputIterator>::value>::type* = 0);
+               typename enable_if<__is_exactly_cpp17_input_iterator<_InputIterator>::value>::type* = 0);
     template <class _InputIterator>
         vector(_InputIterator __first, _InputIterator __last, const allocator_type& __a,
-               typename enable_if<__is_cpp17_input_iterator  <_InputIterator>::value &&
-                                 !__is_cpp17_forward_iterator<_InputIterator>::value>::type* = 0);
+               typename enable_if<__is_exactly_cpp17_input_iterator<_InputIterator>::value>::type* = 0);
     template <class _ForwardIterator>
         vector(_ForwardIterator __first, _ForwardIterator __last,
                typename enable_if<__is_cpp17_forward_iterator<_ForwardIterator>::value>::type* = 0);
@@ -2091,10 +2073,7 @@
         _NOEXCEPT_((__noexcept_move_assign_container<_Allocator, __alloc_traits>::value));
 
     template <class _InputIterator>
-        typename enable_if
-        <
-            __is_cpp17_input_iterator<_InputIterator>::value &&
-           !__is_cpp17_forward_iterator<_InputIterator>::value,
+        typename enable_if <__is_exactly_cpp17_input_iterator<_InputIterator>::value,
            void
         >::type
         assign(_InputIterator __first, _InputIterator __last);
@@ -2206,10 +2185,7 @@
     iterator insert(const_iterator __position, const value_type& __x);
     iterator insert(const_iterator __position, size_type __n, const value_type& __x);
     template <class _InputIterator>
-        typename enable_if
-        <
-             __is_cpp17_input_iterator  <_InputIterator>::value &&
-            !__is_cpp17_forward_iterator<_InputIterator>::value,
+        typename enable_if <__is_exactly_cpp17_input_iterator<_InputIterator>::value,
             iterator
         >::type
         insert(const_iterator __position, _InputIterator __first, _InputIterator __last);
@@ -2516,8 +2492,7 @@
 template <class _Allocator>
 template <class _InputIterator>
 vector<bool, _Allocator>::vector(_InputIterator __first, _InputIterator __last,
-       typename enable_if<__is_cpp17_input_iterator  <_InputIterator>::value &&
-                         !__is_cpp17_forward_iterator<_InputIterator>::value>::type*)
+       typename enable_if<__is_exactly_cpp17_input_iterator<_InputIterator>::value>::type*)
     : __begin_(nullptr),
       __size_(0),
       __cap_alloc_(0, __default_init_tag())
@@ -2543,8 +2518,7 @@
 template <class _Allocator>
 template <class _InputIterator>
 vector<bool, _Allocator>::vector(_InputIterator __first, _InputIterator __last, const allocator_type& __a,
-       typename enable_if<__is_cpp17_input_iterator  <_InputIterator>::value &&
-                         !__is_cpp17_forward_iterator<_InputIterator>::value>::type*)
+       typename enable_if<__is_exactly_cpp17_input_iterator<_InputIterator>::value>::type*)
     : __begin_(nullptr),
       __size_(0),
       __cap_alloc_(0, static_cast<__storage_allocator>(__a))
@@ -2781,10 +2755,7 @@
 
 template <class _Allocator>
 template <class _InputIterator>
-typename enable_if
-<
-    __is_cpp17_input_iterator<_InputIterator>::value &&
-   !__is_cpp17_forward_iterator<_InputIterator>::value,
+typename enable_if <__is_exactly_cpp17_input_iterator<_InputIterator>::value,
    void
 >::type
 vector<bool, _Allocator>::assign(_InputIterator __first, _InputIterator __last)
@@ -2935,10 +2906,7 @@
 
 template <class _Allocator>
 template <class _InputIterator>
-typename enable_if
-<
-     __is_cpp17_input_iterator  <_InputIterator>::value &&
-    !__is_cpp17_forward_iterator<_InputIterator>::value,
+typename enable_if <__is_exactly_cpp17_input_iterator<_InputIterator>::value,
     typename vector<bool, _Allocator>::iterator
 >::type
 vector<bool, _Allocator>::insert(const_iterator __position, _InputIterator __first, _InputIterator __last)