diff --git a/libcxx/docs/FeatureTestMacroTable.rst b/libcxx/docs/FeatureTestMacroTable.rst
--- a/libcxx/docs/FeatureTestMacroTable.rst
+++ b/libcxx/docs/FeatureTestMacroTable.rst
@@ -208,7 +208,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_functional``                ``201907L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_iterator``                  *unimplemented*
+    ``__cpp_lib_constexpr_iterator``                  ``201811L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_memory``                    *unimplemented*
     ------------------------------------------------- -----------------
diff --git a/libcxx/include/iterator b/libcxx/include/iterator
--- a/libcxx/include/iterator
+++ b/libcxx/include/iterator
@@ -152,14 +152,14 @@
     typedef void                        reference;
     typedef void                        pointer;
 
-    explicit back_insert_iterator(Container& x);
-    back_insert_iterator& operator=(const typename Container::value_type& value);
-    back_insert_iterator& operator*();
-    back_insert_iterator& operator++();
-    back_insert_iterator  operator++(int);
+    explicit back_insert_iterator(Container& x);  // constexpr in C++20
+    back_insert_iterator& operator=(const typename Container::value_type& value);  // constexpr in C++20
+    back_insert_iterator& operator*();  // constexpr in C++20
+    back_insert_iterator& operator++();  // constexpr in C++20
+    back_insert_iterator  operator++(int);  // constexpr in C++20
 };
 
-template <class Container> back_insert_iterator<Container> back_inserter(Container& x);
+template <class Container> back_insert_iterator<Container> back_inserter(Container& x);  // constexpr in C++20
 
 template <class Container>
 class front_insert_iterator
@@ -173,14 +173,14 @@
     typedef void                         reference;
     typedef void                         pointer;
 
-    explicit front_insert_iterator(Container& x);
-    front_insert_iterator& operator=(const typename Container::value_type& value);
-    front_insert_iterator& operator*();
-    front_insert_iterator& operator++();
-    front_insert_iterator  operator++(int);
+    explicit front_insert_iterator(Container& x);  // constexpr in C++20
+    front_insert_iterator& operator=(const typename Container::value_type& value);  // constexpr in C++20
+    front_insert_iterator& operator*();  // constexpr in C++20
+    front_insert_iterator& operator++();  // constexpr in C++20
+    front_insert_iterator  operator++(int);  // constexpr in C++20
 };
 
-template <class Container> front_insert_iterator<Container> front_inserter(Container& x);
+template <class Container> front_insert_iterator<Container> front_inserter(Container& x);  // constexpr in C++20
 
 template <class Container>
 class insert_iterator
@@ -195,15 +195,15 @@
     typedef void                   reference;
     typedef void                   pointer;
 
-    insert_iterator(Container& x, typename Container::iterator i);
-    insert_iterator& operator=(const typename Container::value_type& value);
-    insert_iterator& operator*();
-    insert_iterator& operator++();
-    insert_iterator& operator++(int);
+    insert_iterator(Container& x, typename Container::iterator i);  // constexpr in C++20
+    insert_iterator& operator=(const typename Container::value_type& value);  // constexpr in C++20
+    insert_iterator& operator*();  // constexpr in C++20
+    insert_iterator& operator++();  // constexpr in C++20
+    insert_iterator& operator++(int);  // constexpr in C++20
 };
 
 template <class Container, class Iterator>
-insert_iterator<Container> inserter(Container& x, Iterator i);
+insert_iterator<Container> inserter(Container& x, Iterator i);  // constexpr in C++20
 
 template <class Iterator>
 class move_iterator {
@@ -932,20 +932,20 @@
 public:
     typedef _Container container_type;
 
-    _LIBCPP_INLINE_VISIBILITY explicit back_insert_iterator(_Container& __x) : container(_VSTD::addressof(__x)) {}
-    _LIBCPP_INLINE_VISIBILITY back_insert_iterator& operator=(const typename _Container::value_type& __value_)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 explicit back_insert_iterator(_Container& __x) : container(_VSTD::addressof(__x)) {}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator=(const typename _Container::value_type& __value_)
         {container->push_back(__value_); return *this;}
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY back_insert_iterator& operator=(typename _Container::value_type&& __value_)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator=(typename _Container::value_type&& __value_)
         {container->push_back(_VSTD::move(__value_)); return *this;}
 #endif  // _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY back_insert_iterator& operator*()     {return *this;}
-    _LIBCPP_INLINE_VISIBILITY back_insert_iterator& operator++()    {return *this;}
-    _LIBCPP_INLINE_VISIBILITY back_insert_iterator  operator++(int) {return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator*()     {return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator& operator++()    {return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 back_insert_iterator  operator++(int) {return *this;}
 };
 
 template <class _Container>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 back_insert_iterator<_Container>
 back_inserter(_Container& __x)
 {
@@ -965,20 +965,20 @@
 public:
     typedef _Container container_type;
 
-    _LIBCPP_INLINE_VISIBILITY explicit front_insert_iterator(_Container& __x) : container(_VSTD::addressof(__x)) {}
-    _LIBCPP_INLINE_VISIBILITY front_insert_iterator& operator=(const typename _Container::value_type& __value_)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 explicit front_insert_iterator(_Container& __x) : container(_VSTD::addressof(__x)) {}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 front_insert_iterator& operator=(const typename _Container::value_type& __value_)
         {container->push_front(__value_); return *this;}
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY front_insert_iterator& operator=(typename _Container::value_type&& __value_)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 front_insert_iterator& operator=(typename _Container::value_type&& __value_)
         {container->push_front(_VSTD::move(__value_)); return *this;}
 #endif  // _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY front_insert_iterator& operator*()     {return *this;}
-    _LIBCPP_INLINE_VISIBILITY front_insert_iterator& operator++()    {return *this;}
-    _LIBCPP_INLINE_VISIBILITY front_insert_iterator  operator++(int) {return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 front_insert_iterator& operator*()     {return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 front_insert_iterator& operator++()    {return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 front_insert_iterator  operator++(int) {return *this;}
 };
 
 template <class _Container>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 front_insert_iterator<_Container>
 front_inserter(_Container& __x)
 {
@@ -999,21 +999,21 @@
 public:
     typedef _Container container_type;
 
-    _LIBCPP_INLINE_VISIBILITY insert_iterator(_Container& __x, typename _Container::iterator __i)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator(_Container& __x, typename _Container::iterator __i)
         : container(_VSTD::addressof(__x)), iter(__i) {}
-    _LIBCPP_INLINE_VISIBILITY insert_iterator& operator=(const typename _Container::value_type& __value_)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator& operator=(const typename _Container::value_type& __value_)
         {iter = container->insert(iter, __value_); ++iter; return *this;}
 #ifndef _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY insert_iterator& operator=(typename _Container::value_type&& __value_)
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator& operator=(typename _Container::value_type&& __value_)
         {iter = container->insert(iter, _VSTD::move(__value_)); ++iter; return *this;}
 #endif  // _LIBCPP_CXX03_LANG
-    _LIBCPP_INLINE_VISIBILITY insert_iterator& operator*()        {return *this;}
-    _LIBCPP_INLINE_VISIBILITY insert_iterator& operator++()       {return *this;}
-    _LIBCPP_INLINE_VISIBILITY insert_iterator& operator++(int)    {return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator& operator*()        {return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator& operator++()       {return *this;}
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 insert_iterator& operator++(int)    {return *this;}
 };
 
 template <class _Container>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 insert_iterator<_Container>
 inserter(_Container& __x, typename _Container::iterator __i)
 {
diff --git a/libcxx/include/version b/libcxx/include/version
--- a/libcxx/include/version
+++ b/libcxx/include/version
@@ -300,7 +300,7 @@
 // # define __cpp_lib_constexpr_complex                    201711L
 # define __cpp_lib_constexpr_dynamic_alloc              201907L
 # define __cpp_lib_constexpr_functional                 201907L
-// # define __cpp_lib_constexpr_iterator                   201811L
+# define __cpp_lib_constexpr_iterator                   201811L
 // # define __cpp_lib_constexpr_memory                     201811L
 # define __cpp_lib_constexpr_numeric                    201911L
 // # define __cpp_lib_constexpr_string                     201907L
diff --git a/libcxx/test/std/iterators/predef.iterators/insert.iterators/back.insert.iter.ops/back.insert.iter.cons/container.compile.fail.cpp b/libcxx/test/std/iterators/predef.iterators/insert.iterators/back.insert.iter.ops/back.insert.iter.cons/container.compile.fail.cpp
--- a/libcxx/test/std/iterators/predef.iterators/insert.iterators/back.insert.iter.ops/back.insert.iter.cons/container.compile.fail.cpp
+++ b/libcxx/test/std/iterators/predef.iterators/insert.iterators/back.insert.iter.ops/back.insert.iter.cons/container.compile.fail.cpp
@@ -19,7 +19,8 @@
 
 int main(int, char**)
 {
-    std::back_insert_iterator<std::vector<int> > i = std::vector<int>();
+    std::vector<int> v;
+    std::back_insert_iterator<std::vector<int> > i = v;
 
   return 0;
 }
diff --git a/libcxx/test/std/iterators/predef.iterators/insert.iterators/front.insert.iter.ops/front.insert.iter.cons/container.compile.fail.cpp b/libcxx/test/std/iterators/predef.iterators/insert.iterators/front.insert.iter.ops/front.insert.iter.cons/container.compile.fail.cpp
--- a/libcxx/test/std/iterators/predef.iterators/insert.iterators/front.insert.iter.ops/front.insert.iter.cons/container.compile.fail.cpp
+++ b/libcxx/test/std/iterators/predef.iterators/insert.iterators/front.insert.iter.ops/front.insert.iter.cons/container.compile.fail.cpp
@@ -19,7 +19,8 @@
 
 int main(int, char**)
 {
-    std::front_insert_iterator<std::list<int> > i = std::list<int>();
+    std::list<int> l;
+    std::front_insert_iterator<std::list<int> > i = l;
 
   return 0;
 }
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/iterator.version.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/iterator.version.pass.cpp
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/iterator.version.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/iterator.version.pass.cpp
@@ -146,17 +146,11 @@
 #   error "__cpp_lib_array_constexpr should have the value 201811L in c++20"
 # endif
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_constexpr_iterator
-#     error "__cpp_lib_constexpr_iterator should be defined in c++20"
-#   endif
-#   if __cpp_lib_constexpr_iterator != 201811L
-#     error "__cpp_lib_constexpr_iterator should have the value 201811L in c++20"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_constexpr_iterator
-#     error "__cpp_lib_constexpr_iterator should not be defined because it is unimplemented in libc++!"
-#   endif
+# ifndef __cpp_lib_constexpr_iterator
+#   error "__cpp_lib_constexpr_iterator should be defined in c++20"
+# endif
+# if __cpp_lib_constexpr_iterator != 201811L
+#   error "__cpp_lib_constexpr_iterator should have the value 201811L in c++20"
 # endif
 
 # ifndef __cpp_lib_make_reverse_iterator
@@ -209,17 +203,11 @@
 #   error "__cpp_lib_array_constexpr should have the value 201811L in c++2b"
 # endif
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_constexpr_iterator
-#     error "__cpp_lib_constexpr_iterator should be defined in c++2b"
-#   endif
-#   if __cpp_lib_constexpr_iterator != 201811L
-#     error "__cpp_lib_constexpr_iterator should have the value 201811L in c++2b"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_constexpr_iterator
-#     error "__cpp_lib_constexpr_iterator should not be defined because it is unimplemented in libc++!"
-#   endif
+# ifndef __cpp_lib_constexpr_iterator
+#   error "__cpp_lib_constexpr_iterator should be defined in c++2b"
+# endif
+# if __cpp_lib_constexpr_iterator != 201811L
+#   error "__cpp_lib_constexpr_iterator should have the value 201811L in c++2b"
 # endif
 
 # ifndef __cpp_lib_make_reverse_iterator
diff --git a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp
--- a/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp
+++ b/libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp
@@ -2429,17 +2429,11 @@
 #   error "__cpp_lib_constexpr_functional should have the value 201907L in c++20"
 # endif
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_constexpr_iterator
-#     error "__cpp_lib_constexpr_iterator should be defined in c++20"
-#   endif
-#   if __cpp_lib_constexpr_iterator != 201811L
-#     error "__cpp_lib_constexpr_iterator should have the value 201811L in c++20"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_constexpr_iterator
-#     error "__cpp_lib_constexpr_iterator should not be defined because it is unimplemented in libc++!"
-#   endif
+# ifndef __cpp_lib_constexpr_iterator
+#   error "__cpp_lib_constexpr_iterator should be defined in c++20"
+# endif
+# if __cpp_lib_constexpr_iterator != 201811L
+#   error "__cpp_lib_constexpr_iterator should have the value 201811L in c++20"
 # endif
 
 # if !defined(_LIBCPP_VERSION)
@@ -3648,17 +3642,11 @@
 #   error "__cpp_lib_constexpr_functional should have the value 201907L in c++2b"
 # endif
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_constexpr_iterator
-#     error "__cpp_lib_constexpr_iterator should be defined in c++2b"
-#   endif
-#   if __cpp_lib_constexpr_iterator != 201811L
-#     error "__cpp_lib_constexpr_iterator should have the value 201811L in c++2b"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_constexpr_iterator
-#     error "__cpp_lib_constexpr_iterator should not be defined because it is unimplemented in libc++!"
-#   endif
+# ifndef __cpp_lib_constexpr_iterator
+#   error "__cpp_lib_constexpr_iterator should be defined in c++2b"
+# endif
+# if __cpp_lib_constexpr_iterator != 201811L
+#   error "__cpp_lib_constexpr_iterator should have the value 201811L in c++2b"
 # endif
 
 # if !defined(_LIBCPP_VERSION)
diff --git a/libcxx/utils/generate_feature_test_macro_components.py b/libcxx/utils/generate_feature_test_macro_components.py
--- a/libcxx/utils/generate_feature_test_macro_components.py
+++ b/libcxx/utils/generate_feature_test_macro_components.py
@@ -199,7 +199,6 @@
     "name": "__cpp_lib_constexpr_iterator",
     "values": { "c++20": 201811 },
     "headers": ["iterator"],
-    "unimplemented": True,
   }, {
     "name": "__cpp_lib_constexpr_memory",
     "values": { "c++20": 201811 },