diff --git a/libcxx/include/iterator b/libcxx/include/iterator
--- a/libcxx/include/iterator
+++ b/libcxx/include/iterator
@@ -53,6 +53,10 @@
 template<class In>
   concept indirectly_readable = see below;                // since C++20
 
+// [iterator.concept.writable], concept indirectly_­writable
+template<class Out, class T>
+  concept indirectly_writable = see below;                // since C++20
+
 template<class Category, class T, class Distance = ptrdiff_t,
          class Pointer = T*, class Reference = T&>
 struct iterator
@@ -2535,6 +2539,16 @@
 template<class _In>
 concept indirectly_readable = __indirectly_readable_impl<remove_cvref_t<_In>>;
 
+// [iterator.concept.writable]
+template<class _Out, class _Tp>
+concept indirectly_writable =
+  requires(_Out&& __o, _Tp&& __t) {
+    *__o = _VSTD::forward<_Tp>(__t);                        // not required to be equality-preserving
+    *_VSTD::forward<_Out>(__o) = _VSTD::forward<_Tp>(__t);  // not required to be equality-preserving
+    const_cast<const iter_reference_t<_Out>&&>(*__o) = _VSTD::forward<_Tp>(__t);                       // not required to be equality-preserving
+    const_cast<const iter_reference_t<_Out>&&>(*_VSTD::forward<_Out>(__o)) = _VSTD::forward<_Tp>(__t); // not required to be equality-preserving
+  };
+
 #undef _LIBCPP_NOEXCEPT_RETURN
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)
 
diff --git a/libcxx/test/std/containers/associative/map/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/associative/map/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/associative/map/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/associative/map/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,16 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::map<int, int>::iterator>);
+static_assert(!std::indirectly_writable<std::map<int, int>::iterator, std::map<int, int>::iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::map<int, int>::const_iterator>);
+static_assert(
+    !std::indirectly_writable<std::map<int, int>::const_iterator, std::map<int, int>::const_iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::map<int, int>::reverse_iterator>);
+static_assert(
+    !std::indirectly_writable<std::map<int, int>::reverse_iterator, std::map<int, int>::reverse_iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::map<int, int>::const_reverse_iterator>);
+static_assert(!std::indirectly_writable<std::map<int, int>::const_reverse_iterator,
+                                        std::map<int, int>::const_reverse_iterator::value_type>);
diff --git a/libcxx/test/std/containers/associative/multimap/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/associative/multimap/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/associative/multimap/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/associative/multimap/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,17 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::multimap<int, int>::iterator>);
+static_assert(
+    !std::indirectly_writable<std::multimap<int, int>::iterator, std::multimap<int, int>::iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::multimap<int, int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::multimap<int, int>::const_iterator,
+                                        std::multimap<int, int>::const_iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::multimap<int, int>::reverse_iterator>);
+static_assert(!std::indirectly_writable<std::multimap<int, int>::reverse_iterator,
+                                        std::multimap<int, int>::reverse_iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::multimap<int, int>::const_reverse_iterator>);
+static_assert(!std::indirectly_writable<std::multimap<int, int>::const_reverse_iterator,
+                                        std::multimap<int, int>::const_reverse_iterator::value_type>);
diff --git a/libcxx/test/std/containers/associative/multiset/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/associative/multiset/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/associative/multiset/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/associative/multiset/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,13 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::multiset<int>::iterator>);
+static_assert(!std::indirectly_writable<std::multiset<int>::iterator, int>);
+
 static_assert(std::indirectly_readable<std::multiset<int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::multiset<int>::const_iterator, int>);
+
 static_assert(std::indirectly_readable<std::multiset<int>::reverse_iterator>);
+static_assert(!std::indirectly_writable<std::multiset<int>::reverse_iterator, int>);
+
 static_assert(std::indirectly_readable<std::multiset<int>::const_reverse_iterator>);
+static_assert(!std::indirectly_writable<std::multiset<int>::const_reverse_iterator, int>);
diff --git a/libcxx/test/std/containers/associative/set/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/associative/set/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/associative/set/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/associative/set/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,13 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::set<int>::iterator>);
+static_assert(!std::indirectly_writable<std::set<int>::iterator, int>);
+
 static_assert(std::indirectly_readable<std::set<int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::set<int>::const_iterator, int>);
+
 static_assert(std::indirectly_readable<std::set<int>::reverse_iterator>);
+static_assert(!std::indirectly_writable<std::set<int>::reverse_iterator, int>);
+
 static_assert(std::indirectly_readable<std::set<int>::const_reverse_iterator>);
+static_assert(!std::indirectly_writable<std::set<int>::const_reverse_iterator, int>);
diff --git a/libcxx/test/std/containers/sequences/array/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/sequences/array/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/sequences/array/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/sequences/array/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,13 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::array<int, 10>::iterator>);
+static_assert(std::indirectly_writable<std::array<int, 10>::iterator, int>);
+
 static_assert(std::indirectly_readable<std::array<int, 10>::const_iterator>);
+static_assert(!std::indirectly_writable<std::array<int, 10>::const_iterator, int>);
+
 static_assert(std::indirectly_readable<std::array<int, 10>::reverse_iterator>);
+static_assert(std::indirectly_writable<std::array<int, 10>::reverse_iterator, int>);
+
 static_assert(std::indirectly_readable<std::array<int, 10>::const_reverse_iterator>);
+static_assert(!std::indirectly_writable<std::array<int, 10>::const_reverse_iterator, int>);
diff --git a/libcxx/test/std/containers/sequences/deque/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/sequences/deque/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/sequences/deque/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/sequences/deque/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,13 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::deque<int>::iterator>);
+static_assert(std::indirectly_writable<std::deque<int>::iterator, int>);
+
 static_assert(std::indirectly_readable<std::deque<int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::deque<int>::const_iterator, int>);
+
 static_assert(std::indirectly_readable<std::deque<int>::reverse_iterator>);
+static_assert(std::indirectly_writable<std::deque<int>::reverse_iterator, int>);
+
 static_assert(std::indirectly_readable<std::deque<int>::const_reverse_iterator>);
+static_assert(!std::indirectly_writable<std::deque<int>::const_reverse_iterator, int>);
diff --git a/libcxx/test/std/containers/sequences/forwardlist/forwardlist.iter/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/sequences/forwardlist/forwardlist.iter/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/sequences/forwardlist/forwardlist.iter/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/sequences/forwardlist/forwardlist.iter/iterator_concept_conformance.compile.pass.cpp
@@ -17,4 +17,7 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::forward_list<int>::iterator>);
+static_assert(std::indirectly_writable<std::forward_list<int>::iterator, int>);
+
 static_assert(std::indirectly_readable<std::forward_list<int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::forward_list<int>::const_iterator, int>);
diff --git a/libcxx/test/std/containers/sequences/list/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/sequences/list/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/sequences/list/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/sequences/list/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,13 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::list<int>::iterator>);
+static_assert(std::indirectly_writable<std::list<int>::iterator, int>);
+
 static_assert(std::indirectly_readable<std::list<int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::list<int>::const_iterator, int>);
+
 static_assert(std::indirectly_readable<std::list<int>::reverse_iterator>);
+static_assert(std::indirectly_writable<std::list<int>::reverse_iterator, int>);
+
 static_assert(std::indirectly_readable<std::list<int>::const_reverse_iterator>);
+static_assert(!std::indirectly_writable<std::list<int>::const_reverse_iterator, int>);
diff --git a/libcxx/test/std/containers/sequences/vector/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/sequences/vector/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/sequences/vector/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/sequences/vector/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,13 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::vector<int>::iterator>);
+static_assert(std::indirectly_writable<std::vector<int>::iterator, int>);
+
 static_assert(std::indirectly_readable<std::vector<int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::vector<int>::const_iterator, int>);
+
 static_assert(std::indirectly_readable<std::vector<int>::reverse_iterator>);
+static_assert(std::indirectly_writable<std::vector<int>::reverse_iterator, int>);
+
 static_assert(std::indirectly_readable<std::vector<int>::const_reverse_iterator>);
+static_assert(!std::indirectly_writable<std::vector<int>::const_reverse_iterator, int>);
diff --git a/libcxx/test/std/containers/unord/unord.map/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/unord/unord.map/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/unord/unord.map/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/unord/unord.map/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,17 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::unordered_map<int, int>::iterator>);
+static_assert(!std::indirectly_writable<std::unordered_map<int, int>::iterator,
+                                        std::unordered_map<int, int>::iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::unordered_map<int, int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_map<int, int>::const_iterator,
+                                        std::unordered_map<int, int>::const_iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::unordered_map<int, int>::local_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_map<int, int>::local_iterator,
+                                        std::unordered_map<int, int>::local_iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::unordered_map<int, int>::const_local_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_map<int, int>::const_local_iterator,
+                                        std::unordered_map<int, int>::const_local_iterator::value_type>);
diff --git a/libcxx/test/std/containers/unord/unord.multimap/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/unord/unord.multimap/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/unord/unord.multimap/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/unord/unord.multimap/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,17 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::unordered_multimap<int, int>::iterator>);
+static_assert(!std::indirectly_writable<std::unordered_multimap<int, int>::iterator,
+                                        std::unordered_multimap<int, int>::iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::unordered_multimap<int, int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_multimap<int, int>::const_iterator,
+                                        std::unordered_multimap<int, int>::const_iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::unordered_multimap<int, int>::local_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_multimap<int, int>::local_iterator,
+                                        std::unordered_multimap<int, int>::local_iterator::value_type>);
+
 static_assert(std::indirectly_readable<std::unordered_multimap<int, int>::const_local_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_multimap<int, int>::const_local_iterator,
+                                        std::unordered_multimap<int, int>::const_local_iterator::value_type>);
diff --git a/libcxx/test/std/containers/unord/unord.multiset/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/unord/unord.multiset/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/unord/unord.multiset/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/unord/unord.multiset/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,13 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::unordered_multiset<int>::iterator>);
+static_assert(!std::indirectly_writable<std::unordered_multiset<int>::iterator, int>);
+
 static_assert(std::indirectly_readable<std::unordered_multiset<int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_multiset<int>::const_iterator, int>);
+
 static_assert(std::indirectly_readable<std::unordered_multiset<int>::local_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_multiset<int>::local_iterator, int>);
+
 static_assert(std::indirectly_readable<std::unordered_multiset<int>::const_local_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_multiset<int>::const_local_iterator, int>);
diff --git a/libcxx/test/std/containers/unord/unord.set/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/unord/unord.set/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/unord/unord.set/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/unord/unord.set/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,13 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::unordered_set<int>::iterator>);
+static_assert(!std::indirectly_writable<std::unordered_set<int>::iterator, int>);
+
 static_assert(std::indirectly_readable<std::unordered_set<int>::const_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_set<int>::const_iterator, int>);
+
 static_assert(std::indirectly_readable<std::unordered_set<int>::local_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_set<int>::local_iterator, int>);
+
 static_assert(std::indirectly_readable<std::unordered_set<int>::const_local_iterator>);
+static_assert(!std::indirectly_writable<std::unordered_set<int>::const_local_iterator, int>);
diff --git a/libcxx/test/std/containers/views/span.iterators/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/containers/views/span.iterators/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/containers/views/span.iterators/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/containers/views/span.iterators/iterator_concept_conformance.compile.pass.cpp
@@ -17,4 +17,7 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::span<int>::iterator>);
+static_assert(std::indirectly_writable<std::span<int>::iterator, int>);
+
 static_assert(std::indirectly_readable<std::span<int>::reverse_iterator>);
+static_assert(std::indirectly_writable<std::span<int>::reverse_iterator, int>);
diff --git a/libcxx/test/std/input.output/filesystems/class.directory_iterator/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/input.output/filesystems/class.directory_iterator/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/input.output/filesystems/class.directory_iterator/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/input.output/filesystems/class.directory_iterator/iterator_concept_conformance.compile.pass.cpp
@@ -17,4 +17,8 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<fs::directory_iterator>);
+static_assert(!std::indirectly_writable<fs::directory_iterator, fs::directory_iterator::value_type>);
+
 static_assert(std::indirectly_readable<fs::recursive_directory_iterator>);
+static_assert(
+    !std::indirectly_writable<fs::recursive_directory_iterator, fs::recursive_directory_iterator::value_type>);
diff --git a/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.writable/indirectly_writable.compile.pass.cpp b/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.writable/indirectly_writable.compile.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/iterator.requirements/iterator.concepts/iterator.concept.writable/indirectly_writable.compile.pass.cpp
@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// template<class In>
+// concept indirectly_writable;
+
+#include <iterator>
+
+#include <concepts>
+#include <memory>
+#include <optional>
+
+#include "read_write.h"
+
+template <class Out, class T>
+[[nodiscard]] constexpr bool check_indirectly_writable() {
+  constexpr bool result = std::indirectly_writable<Out, T>;
+  static_assert(std::indirectly_writable<Out const, T> == result);
+  return result;
+}
+
+static_assert(check_indirectly_writable<value_type_indirection, int>());
+static_assert(check_indirectly_writable<value_type_indirection, double>());
+static_assert(!check_indirectly_writable<value_type_indirection, double*>());
+
+static_assert(!check_indirectly_writable<read_only_indirection, int>());
+static_assert(!check_indirectly_writable<proxy_indirection, int>());
+
+static_assert(!check_indirectly_writable<int, int>());
+static_assert(!check_indirectly_writable<missing_dereference, missing_dereference::value_type>());
+
+namespace standard_types {
+static_assert(!check_indirectly_writable<void*, int>());
+static_assert(!check_indirectly_writable<void const*, int>());
+static_assert(!check_indirectly_writable<void volatile*, int>());
+static_assert(!check_indirectly_writable<void const volatile*, int>());
+static_assert(!check_indirectly_writable<void*, double>());
+static_assert(check_indirectly_writable<void**, int*>());
+static_assert(!check_indirectly_writable<void**, int>());
+
+static_assert(check_indirectly_writable<int*, int>());
+static_assert(!check_indirectly_writable<int const*, int>());
+static_assert(check_indirectly_writable<int volatile*, int>());
+static_assert(!check_indirectly_writable<int const volatile*, int>());
+static_assert(check_indirectly_writable<int*, double>());
+static_assert(check_indirectly_writable<int**, int*>());
+static_assert(!check_indirectly_writable<int**, int>());
+
+// <memory>
+static_assert(check_indirectly_writable<std::shared_ptr<int>, int>());
+static_assert(!check_indirectly_writable<std::shared_ptr<void>, int>());
+static_assert(check_indirectly_writable<std::unique_ptr<int>, int>());
+static_assert(!check_indirectly_writable<std::unique_ptr<void>, int>());
+
+// <optional>
+static_assert(!check_indirectly_writable<std::optional<int>, int>());
+} // namespace standard_types
diff --git a/libcxx/test/std/iterators/predef.iterators/insert.iterators/back.insert.iterator/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/iterators/predef.iterators/insert.iterators/back.insert.iterator/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/iterators/predef.iterators/insert.iterators/back.insert.iterator/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/iterators/predef.iterators/insert.iterators/back.insert.iterator/iterator_concept_conformance.compile.pass.cpp
@@ -17,3 +17,4 @@
 #include <vector>
 
 static_assert(!std::indirectly_readable<std::back_insert_iterator<std::vector<int> > >);
+static_assert(std::indirectly_writable<std::back_insert_iterator<std::vector<int> >, int>);
diff --git a/libcxx/test/std/iterators/predef.iterators/insert.iterators/front.insert.iterator/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/iterators/predef.iterators/insert.iterators/front.insert.iterator/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/iterators/predef.iterators/insert.iterators/front.insert.iterator/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/iterators/predef.iterators/insert.iterators/front.insert.iterator/iterator_concept_conformance.compile.pass.cpp
@@ -17,3 +17,4 @@
 #include <list>
 
 static_assert(!std::indirectly_readable<std::front_insert_iterator<std::list<int> > >);
+static_assert(std::indirectly_writable<std::front_insert_iterator<std::list<int> >, int>);
diff --git a/libcxx/test/std/iterators/predef.iterators/insert.iterators/insert.iterator/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/iterators/predef.iterators/insert.iterators/insert.iterator/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/iterators/predef.iterators/insert.iterators/insert.iterator/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/iterators/predef.iterators/insert.iterators/insert.iterator/iterator_concept_conformance.compile.pass.cpp
@@ -17,3 +17,4 @@
 #include <vector>
 
 static_assert(!std::indirectly_readable<std::insert_iterator<std::vector<int> > >);
+static_assert(std::indirectly_writable<std::insert_iterator<std::vector<int> >, int>);
diff --git a/libcxx/test/std/iterators/predef.iterators/move.iterators/move.iterator/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/iterators/predef.iterators/move.iterators/move.iterator/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/iterators/predef.iterators/move.iterators/move.iterator/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/iterators/predef.iterators/move.iterators/move.iterator/iterator_concept_conformance.compile.pass.cpp
@@ -15,3 +15,4 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::move_iterator<int*> >);
+static_assert(!std::indirectly_writable<std::move_iterator<int*>, int>);
diff --git a/libcxx/test/std/iterators/stream.iterators/istream.iterator/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/iterators/stream.iterators/istream.iterator/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/iterators/stream.iterators/istream.iterator/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/iterators/stream.iterators/istream.iterator/iterator_concept_conformance.compile.pass.cpp
@@ -17,3 +17,4 @@
 #include <iosfwd>
 
 static_assert(std::indirectly_readable<std::istream_iterator<int, std::istream> >);
+static_assert(!std::indirectly_writable<std::istream_iterator<int, std::istream>, int>);
diff --git a/libcxx/test/std/iterators/stream.iterators/istreambuf.iterator/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/iterators/stream.iterators/istreambuf.iterator/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/iterators/stream.iterators/istreambuf.iterator/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/iterators/stream.iterators/istreambuf.iterator/iterator_concept_conformance.compile.pass.cpp
@@ -18,3 +18,4 @@
 #include <string>
 
 static_assert(std::indirectly_readable<std::istreambuf_iterator<char> >);
+static_assert(!std::indirectly_writable<std::istreambuf_iterator<char>, char>);
diff --git a/libcxx/test/std/iterators/stream.iterators/ostream.iterator/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/iterators/stream.iterators/ostream.iterator/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/iterators/stream.iterators/ostream.iterator/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/iterators/stream.iterators/ostream.iterator/iterator_concept_conformance.compile.pass.cpp
@@ -17,3 +17,4 @@
 #include <iosfwd>
 
 static_assert(!std::indirectly_readable<std::ostream_iterator<int, std::ostream> >);
+static_assert(std::indirectly_writable<std::ostream_iterator<int, std::ostream>, int>);
diff --git a/libcxx/test/std/iterators/stream.iterators/ostreambuf.iterator/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/iterators/stream.iterators/ostreambuf.iterator/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/iterators/stream.iterators/ostreambuf.iterator/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/iterators/stream.iterators/ostreambuf.iterator/iterator_concept_conformance.compile.pass.cpp
@@ -18,3 +18,4 @@
 #include <string>
 
 static_assert(!std::indirectly_readable<std::ostreambuf_iterator<char> >);
+static_assert(std::indirectly_writable<std::ostreambuf_iterator<char>, char>);
diff --git a/libcxx/test/std/re/re.iter/re.regiter/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/re/re.iter/re.regiter/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/re/re.iter/re.regiter/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/re/re.iter/re.regiter/iterator_concept_conformance.compile.pass.cpp
@@ -16,7 +16,6 @@
 
 #include <iterator>
 
-static_assert(std::indirectly_readable<std::regex_iterator<std::string_view::iterator> >);
-static_assert(std::indirectly_readable<std::regex_iterator<std::string_view::const_iterator> >);
-static_assert(std::indirectly_readable<std::regex_iterator<std::string_view::reverse_iterator> >);
-static_assert(std::indirectly_readable<std::regex_iterator<std::string_view::const_reverse_iterator> >);
+using iterator = std::regex_iterator<char*>;
+static_assert(std::indirectly_readable<iterator>);
+static_assert(!std::indirectly_writable<iterator, iterator::value_type>);
diff --git a/libcxx/test/std/re/re.iter/re.tokiter/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/re/re.iter/re.tokiter/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/re/re.iter/re.tokiter/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/re/re.iter/re.tokiter/iterator_concept_conformance.compile.pass.cpp
@@ -16,7 +16,6 @@
 
 #include <iterator>
 
-static_assert(std::indirectly_readable<std::regex_token_iterator<std::string_view::iterator> >);
-static_assert(std::indirectly_readable<std::regex_token_iterator<std::string_view::const_iterator> >);
-static_assert(std::indirectly_readable<std::regex_token_iterator<std::string_view::reverse_iterator> >);
-static_assert(std::indirectly_readable<std::regex_token_iterator<std::string_view::const_reverse_iterator> >);
+using iterator = std::regex_token_iterator<char*>;
+static_assert(std::indirectly_readable<iterator>);
+static_assert(!std::indirectly_writable<iterator, iterator::value_type>);
diff --git a/libcxx/test/std/strings/basic.string/string.iterators/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/strings/basic.string/string.iterators/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/strings/basic.string/string.iterators/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/strings/basic.string/string.iterators/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,13 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::string::iterator>);
+static_assert(std::indirectly_writable<std::string::iterator, char>);
+
 static_assert(std::indirectly_readable<std::string::const_iterator>);
+static_assert(!std::indirectly_writable<std::string::const_iterator, char>);
+
 static_assert(std::indirectly_readable<std::string::reverse_iterator>);
+static_assert(std::indirectly_writable<std::string::reverse_iterator, char>);
+
 static_assert(std::indirectly_readable<std::string::const_reverse_iterator>);
+static_assert(!std::indirectly_writable<std::string::const_reverse_iterator, char>);
diff --git a/libcxx/test/std/strings/string.view/string.view.iterators/iterator_concept_conformance.compile.pass.cpp b/libcxx/test/std/strings/string.view/string.view.iterators/iterator_concept_conformance.compile.pass.cpp
--- a/libcxx/test/std/strings/string.view/string.view.iterators/iterator_concept_conformance.compile.pass.cpp
+++ b/libcxx/test/std/strings/string.view/string.view.iterators/iterator_concept_conformance.compile.pass.cpp
@@ -17,6 +17,13 @@
 #include <iterator>
 
 static_assert(std::indirectly_readable<std::string_view::iterator>);
+static_assert(!std::indirectly_writable<std::string_view::iterator, char>);
+
 static_assert(std::indirectly_readable<std::string_view::const_iterator>);
+static_assert(!std::indirectly_writable<std::string_view::const_iterator, char>);
+
 static_assert(std::indirectly_readable<std::string_view::reverse_iterator>);
+static_assert(!std::indirectly_writable<std::string_view::reverse_iterator, char>);
+
 static_assert(std::indirectly_readable<std::string_view::const_reverse_iterator>);
+static_assert(!std::indirectly_writable<std::string_view::const_reverse_iterator, char>);