diff --git a/libcxx/include/__algorithm/in_in_out_result.h b/libcxx/include/__algorithm/in_in_out_result.h
--- a/libcxx/include/__algorithm/in_in_out_result.h
+++ b/libcxx/include/__algorithm/in_in_out_result.h
@@ -26,9 +26,9 @@
 
 template <class _I1, class _I2, class _O1>
 struct in_in_out_result {
-  [[no_unique_address]] _I1 in1;
-  [[no_unique_address]] _I2 in2;
-  [[no_unique_address]] _O1 out;
+  _LIBCPP_NO_UNIQUE_ADDRESS _I1 in1;
+  _LIBCPP_NO_UNIQUE_ADDRESS _I2 in2;
+  _LIBCPP_NO_UNIQUE_ADDRESS _O1 out;
 
   template <class _II1, class _II2, class _OO1>
     requires convertible_to<const _I1&, _II1> && convertible_to<const _I2&, _II2> && convertible_to<const _O1&, _OO1>
diff --git a/libcxx/include/__algorithm/in_in_result.h b/libcxx/include/__algorithm/in_in_result.h
--- a/libcxx/include/__algorithm/in_in_result.h
+++ b/libcxx/include/__algorithm/in_in_result.h
@@ -26,8 +26,8 @@
 
 template <class _I1, class _I2>
 struct in_in_result {
-  [[no_unique_address]] _I1 in1;
-  [[no_unique_address]] _I2 in2;
+  _LIBCPP_NO_UNIQUE_ADDRESS _I1 in1;
+  _LIBCPP_NO_UNIQUE_ADDRESS _I2 in2;
 
   template <class _II1, class _II2>
     requires convertible_to<const _I1&, _II1> && convertible_to<const _I2&, _II2>
diff --git a/libcxx/include/__algorithm/in_out_out_result.h b/libcxx/include/__algorithm/in_out_out_result.h
--- a/libcxx/include/__algorithm/in_out_out_result.h
+++ b/libcxx/include/__algorithm/in_out_out_result.h
@@ -25,9 +25,9 @@
 namespace ranges {
 template <class _I1, class _O1, class _O2>
 struct in_out_out_result {
-  [[no_unique_address]] _I1 in;
-  [[no_unique_address]] _O1 out1;
-  [[no_unique_address]] _O2 out2;
+  _LIBCPP_NO_UNIQUE_ADDRESS _I1 in;
+  _LIBCPP_NO_UNIQUE_ADDRESS _O1 out1;
+  _LIBCPP_NO_UNIQUE_ADDRESS _O2 out2;
 
   template <class _II1, class _OO1, class _OO2>
     requires convertible_to<const _I1&, _II1> && convertible_to<const _O1&, _OO1> && convertible_to<const _O2&, _OO2>
diff --git a/libcxx/include/__algorithm/in_out_result.h b/libcxx/include/__algorithm/in_out_result.h
--- a/libcxx/include/__algorithm/in_out_result.h
+++ b/libcxx/include/__algorithm/in_out_result.h
@@ -26,8 +26,8 @@
 
 template<class _InputIterator, class _OutputIterator>
 struct in_out_result {
-  [[no_unique_address]] _InputIterator in;
-  [[no_unique_address]] _OutputIterator out;
+  _LIBCPP_NO_UNIQUE_ADDRESS _InputIterator in;
+  _LIBCPP_NO_UNIQUE_ADDRESS _OutputIterator out;
 
   template <class _InputIterator2, class _OutputIterator2>
     requires convertible_to<const _InputIterator&, _InputIterator2> && convertible_to<const _OutputIterator&,
diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1398,6 +1398,25 @@
 # define _LIBCPP_ATTRIBUTE_FORMAT(archetype, format_string_index, first_format_arg_index) /* nothing */
 #endif
 
+#if __has_cpp_attribute(no_unique_address) && !defined(_MSC_VER)
+   // MSVC implements [[no_unique_address]] as a silent no-op currently.
+   // If/when MSVC breaks its C++ ABI, it will be changed to work as intended.
+   // Clang-cl does not yet (14.0) implement no_unique_address at all, so
+   // any use of this attribute triggers a warning (and doesn't give the
+   // intended effect).
+#  define _LIBCPP_NO_UNIQUE_ADDRESS [[no_unique_address]]
+#elif __has_cpp_attribute(msvc::no_unique_address)
+   // MSVC does implement [[msvc::no_unique_address]] which does what
+   // [[no_unique_address]] is supposed to do, in general.
+   // Clang-cl does not yet (14.0) implement msvc::no_unique_address though.
+#  define _LIBCPP_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
+#else
+#  define _LIBCPP_NO_UNIQUE_ADDRESS /* nothing */
+   // Note that this can be replaced by #error as soon as clang-cl
+   // implements msvc::no_unique_address, since there should be no C++20
+   // compiler that doesn't support one of the two attributes at that point.
+#endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG
diff --git a/libcxx/include/__iterator/counted_iterator.h b/libcxx/include/__iterator/counted_iterator.h
--- a/libcxx/include/__iterator/counted_iterator.h
+++ b/libcxx/include/__iterator/counted_iterator.h
@@ -65,7 +65,7 @@
   , public __counted_iterator_value_type<_Iter>
 {
 public:
-  [[no_unique_address]] _Iter __current_ = _Iter();
+  _LIBCPP_NO_UNIQUE_ADDRESS _Iter __current_ = _Iter();
   iter_difference_t<_Iter> __count_ = 0;
 
   using iterator_type = _Iter;
diff --git a/libcxx/include/__ranges/copyable_box.h b/libcxx/include/__ranges/copyable_box.h
--- a/libcxx/include/__ranges/copyable_box.h
+++ b/libcxx/include/__ranges/copyable_box.h
@@ -41,7 +41,7 @@
   // Primary template - uses std::optional and introduces an empty state in case assignment fails.
   template<__copy_constructible_object _Tp>
   class __copyable_box {
-    [[no_unique_address]] optional<_Tp> __val_;
+    _LIBCPP_NO_UNIQUE_ADDRESS optional<_Tp> __val_;
 
   public:
     template<class ..._Args>
@@ -116,7 +116,7 @@
   template<__copy_constructible_object _Tp>
     requires __doesnt_need_empty_state_for_copy<_Tp> && __doesnt_need_empty_state_for_move<_Tp>
   class __copyable_box<_Tp> {
-    [[no_unique_address]] _Tp __val_;
+    _LIBCPP_NO_UNIQUE_ADDRESS _Tp __val_;
 
   public:
     template<class ..._Args>
diff --git a/libcxx/include/__ranges/drop_view.h b/libcxx/include/__ranges/drop_view.h
--- a/libcxx/include/__ranges/drop_view.h
+++ b/libcxx/include/__ranges/drop_view.h
@@ -45,7 +45,7 @@
     // one can't call begin() on it more than once.
     static constexpr bool _UseCache = forward_range<_View> && !(random_access_range<_View> && sized_range<_View>);
     using _Cache = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
-    [[no_unique_address]] _Cache __cached_begin_ = _Cache();
+    _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
     range_difference_t<_View> __count_ = 0;
     _View __base_ = _View();
 
diff --git a/libcxx/include/__ranges/join_view.h b/libcxx/include/__ranges/join_view.h
--- a/libcxx/include/__ranges/join_view.h
+++ b/libcxx/include/__ranges/join_view.h
@@ -67,8 +67,8 @@
 
     static constexpr bool _UseCache = !is_reference_v<_InnerRange>;
     using _Cache = _If<_UseCache, __non_propagating_cache<remove_cvref_t<_InnerRange>>, __empty_cache>;
-    [[no_unique_address]] _Cache __cache_;
-    _View __base_ = _View(); // TODO: [[no_unique_address]] makes clang crash! File a bug :)
+    _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cache_;
+    _View __base_ = _View(); // TODO: _LIBCPP_NO_UNIQUE_ADDRESS makes clang crash! File a bug :)
 
   public:
     _LIBCPP_HIDE_FROM_ABI
diff --git a/libcxx/include/__ranges/reverse_view.h b/libcxx/include/__ranges/reverse_view.h
--- a/libcxx/include/__ranges/reverse_view.h
+++ b/libcxx/include/__ranges/reverse_view.h
@@ -43,8 +43,8 @@
     // amortized O(1) begin() method.
     static constexpr bool _UseCache = !random_access_range<_View> && !common_range<_View>;
     using _Cache = _If<_UseCache, __non_propagating_cache<reverse_iterator<iterator_t<_View>>>, __empty_cache>;
-    [[no_unique_address]] _Cache __cached_begin_ = _Cache();
-    [[no_unique_address]] _View __base_ = _View();
+    _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
+    _LIBCPP_NO_UNIQUE_ADDRESS _View __base_ = _View();
 
   public:
     _LIBCPP_HIDE_FROM_ABI
diff --git a/libcxx/include/__ranges/subrange.h b/libcxx/include/__ranges/subrange.h
--- a/libcxx/include/__ranges/subrange.h
+++ b/libcxx/include/__ranges/subrange.h
@@ -82,9 +82,9 @@
     static constexpr bool _MustProvideSizeAtConstruction = !_StoreSize; // just to improve compiler diagnostics
     struct _Empty { constexpr _Empty(auto) noexcept { } };
     using _Size = conditional_t<_StoreSize, make_unsigned_t<iter_difference_t<_Iter>>, _Empty>;
-    [[no_unique_address]] _Iter __begin_ = _Iter();
-    [[no_unique_address]] _Sent __end_ = _Sent();
-    [[no_unique_address]] _Size __size_ = 0;
+    _LIBCPP_NO_UNIQUE_ADDRESS _Iter __begin_ = _Iter();
+    _LIBCPP_NO_UNIQUE_ADDRESS _Sent __end_ = _Sent();
+    _LIBCPP_NO_UNIQUE_ADDRESS _Size __size_ = 0;
 
   public:
     _LIBCPP_HIDE_FROM_ABI
diff --git a/libcxx/include/__ranges/take_view.h b/libcxx/include/__ranges/take_view.h
--- a/libcxx/include/__ranges/take_view.h
+++ b/libcxx/include/__ranges/take_view.h
@@ -39,7 +39,7 @@
 namespace ranges {
   template<view _View>
   class take_view : public view_interface<take_view<_View>> {
-    [[no_unique_address]] _View __base_ = _View();
+    _LIBCPP_NO_UNIQUE_ADDRESS _View __base_ = _View();
     range_difference_t<_View> __count_ = 0;
 
     template<bool> class __sentinel;
@@ -136,7 +136,7 @@
     using _Base = __maybe_const<_Const, _View>;
     template<bool _OtherConst>
     using _Iter = counted_iterator<iterator_t<__maybe_const<_OtherConst, _View>>>;
-    [[no_unique_address]] sentinel_t<_Base> __end_ = sentinel_t<_Base>();
+    _LIBCPP_NO_UNIQUE_ADDRESS sentinel_t<_Base> __end_ = sentinel_t<_Base>();
 
     template<bool>
     friend class take_view<_View>::__sentinel;
diff --git a/libcxx/include/__ranges/transform_view.h b/libcxx/include/__ranges/transform_view.h
--- a/libcxx/include/__ranges/transform_view.h
+++ b/libcxx/include/__ranges/transform_view.h
@@ -61,8 +61,8 @@
   template<bool> class __iterator;
   template<bool> class __sentinel;
 
-  [[no_unique_address]] __copyable_box<_Fn> __func_;
-  [[no_unique_address]] _View __base_ = _View();
+  _LIBCPP_NO_UNIQUE_ADDRESS __copyable_box<_Fn> __func_;
+  _LIBCPP_NO_UNIQUE_ADDRESS _View __base_ = _View();
 
 public:
   _LIBCPP_HIDE_FROM_ABI
diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -571,6 +571,7 @@
     # anyway), thus just disable the experimental library. Remove this
     # setting when cmake and the test driver does the right thing automatically.
     generate-cmake-libcxx-win -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF \
+                              -DLIBCXX_ENABLE_WERROR=ON \
                               -DLIBCXX_TEST_CONFIG="llvm-libc++-shared-clangcl.cfg.in"
     echo "+++ Running the libc++ tests"
     ${NINJA} -vC "${BUILD_DIR}" check-cxx
@@ -578,6 +579,7 @@
 clang-cl-static)
     clean
     generate-cmake-libcxx-win -DLIBCXX_ENABLE_SHARED=OFF \
+                              -DLIBCXX_ENABLE_WERROR=ON \
                               -DLIBCXX_TEST_CONFIG="llvm-libc++-static-clangcl.cfg.in"
     echo "+++ Running the libc++ tests"
     ${NINJA} -vC "${BUILD_DIR}" check-cxx