diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -70,6 +70,15 @@
   from the Standard since it was never used, the proper specialization to use
   instead is ``template<size_t N> struct formatter<charT[N], charT>``.
 
+- Libc++ used to provide some C++11 tag type global variables in C++03 as an extension, which are removed in
+  this release. Those variables were ``std::allocator_arg``, ``std::defer_lock``, ``std::try_to_lock``,
+  ``std::adopt_lock``, and ``std::piecewise_construct``. Note that the types associated to those variables are
+  still provided in C++03 as an extension (e.g. ``std::piecewise_construct_t``). Providing those variables in
+  C++03 mode made it impossible to define them properly -- C++11 mandated that they be ``constexpr`` variables,
+  which is impossible in C++03 mode. Furthermore, C++17 mandated that they be ``inline constexpr`` variables,
+  which led to ODR violations when mixed with the C++03 definition. Cleaning this up is required for libc++ to
+  make progress on support for C++20 modules.
+
 Upcoming Deprecations and Removals
 ----------------------------------
 
diff --git a/libcxx/include/__memory/allocator_arg_t.h b/libcxx/include/__memory/allocator_arg_t.h
--- a/libcxx/include/__memory/allocator_arg_t.h
+++ b/libcxx/include/__memory/allocator_arg_t.h
@@ -25,9 +25,9 @@
 
 struct _LIBCPP_TEMPLATE_VIS allocator_arg_t { explicit allocator_arg_t() = default; };
 
-#if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
+#if defined(_LIBCPP_BUILDING_LIBRARY)
 extern _LIBCPP_EXPORTED_FROM_ABI const allocator_arg_t allocator_arg;
-#else
+#elif !defined(_LIBCPP_CXX03_LANG)
 /* inline */ constexpr allocator_arg_t allocator_arg = allocator_arg_t();
 #endif
 
diff --git a/libcxx/include/__mutex_base b/libcxx/include/__mutex_base
--- a/libcxx/include/__mutex_base
+++ b/libcxx/include/__mutex_base
@@ -64,19 +64,15 @@
 struct _LIBCPP_TYPE_VIS try_to_lock_t { explicit try_to_lock_t() = default; };
 struct _LIBCPP_TYPE_VIS adopt_lock_t { explicit adopt_lock_t() = default; };
 
-#if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
-
-extern _LIBCPP_EXPORTED_FROM_ABI const defer_lock_t  defer_lock;
+#  if defined(_LIBCPP_BUILDING_LIBRARY)
+extern _LIBCPP_EXPORTED_FROM_ABI const defer_lock_t defer_lock;
 extern _LIBCPP_EXPORTED_FROM_ABI const try_to_lock_t try_to_lock;
-extern _LIBCPP_EXPORTED_FROM_ABI const adopt_lock_t  adopt_lock;
-
-#else
-
-/* inline */ constexpr defer_lock_t  defer_lock  = defer_lock_t();
+extern _LIBCPP_EXPORTED_FROM_ABI const adopt_lock_t adopt_lock;
+#  elif !defined(_LIBCPP_CXX03_LANG)
+/* inline */ constexpr defer_lock_t defer_lock = defer_lock_t();
 /* inline */ constexpr try_to_lock_t try_to_lock = try_to_lock_t();
-/* inline */ constexpr adopt_lock_t  adopt_lock  = adopt_lock_t();
-
-#endif
+/* inline */ constexpr adopt_lock_t adopt_lock = adopt_lock_t();
+#  endif
 
 template <class _Mutex>
 class _LIBCPP_TEMPLATE_VIS _LIBCPP_THREAD_SAFETY_ANNOTATION(scoped_lockable)
diff --git a/libcxx/include/__utility/piecewise_construct.h b/libcxx/include/__utility/piecewise_construct.h
--- a/libcxx/include/__utility/piecewise_construct.h
+++ b/libcxx/include/__utility/piecewise_construct.h
@@ -18,9 +18,10 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 struct _LIBCPP_TEMPLATE_VIS piecewise_construct_t { explicit piecewise_construct_t() = default; };
-#if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
-extern _LIBCPP_EXPORTED_FROM_ABI const piecewise_construct_t piecewise_construct;// = piecewise_construct_t();
-#else
+
+#if defined(_LIBCPP_BUILDING_LIBRARY)
+extern _LIBCPP_EXPORTED_FROM_ABI const piecewise_construct_t piecewise_construct;
+#elif !defined(_LIBCPP_CXX03_LANG)
 /* inline */ constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
 #endif