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
@@ -66,19 +66,15 @@
 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
 
diff --git a/libcxx/include/condition_variable b/libcxx/include/condition_variable
--- a/libcxx/include/condition_variable
+++ b/libcxx/include/condition_variable
@@ -202,7 +202,7 @@
     unique_lock<mutex> __lk(*__mut);
     __lock.unlock();
     unique_ptr<_Lock, __lock_external> __lxx(&__lock);
-    lock_guard<unique_lock<mutex> > __lx(__lk, adopt_lock);
+    lock_guard<unique_lock<mutex> > __lx(__lk, adopt_lock_t());
     __cv_.wait(__lk);
 }  // __mut_.unlock(), __lock.lock()
 
@@ -224,7 +224,7 @@
     unique_lock<mutex> __lk(*__mut);
     __lock.unlock();
     unique_ptr<_Lock, __lock_external> __lxx(&__lock);
-    lock_guard<unique_lock<mutex> > __lx(__lk, adopt_lock);
+    lock_guard<unique_lock<mutex> > __lx(__lk, adopt_lock_t());
     return __cv_.wait_until(__lk, __t);
 }  // __mut_.unlock(), __lock.lock()
 
diff --git a/libcxx/include/experimental/__memory b/libcxx/include/experimental/__memory
--- a/libcxx/include/experimental/__memory
+++ b/libcxx/include/experimental/__memory
@@ -93,7 +93,7 @@
 inline _LIBCPP_INLINE_VISIBILITY
 void __user_alloc_construct_impl (integral_constant<int, 1>, _Tp *__storage, const _Allocator &__a, _Args &&... __args )
 {
-    new (__storage) _Tp (allocator_arg, __a, _VSTD::forward<_Args>(__args)...);
+    new (__storage) _Tp (allocator_arg_t(), __a, _VSTD::forward<_Args>(__args)...);
 }
 
 // FIXME: This should have a version which takes a non-const alloc.
diff --git a/libcxx/include/future b/libcxx/include/future
--- a/libcxx/include/future
+++ b/libcxx/include/future
@@ -1906,8 +1906,8 @@
               class = __enable_if_t<!is_same<__remove_cvref_t<_Fp>, packaged_task>::value> >
         _LIBCPP_INLINE_VISIBILITY
         packaged_task(allocator_arg_t, const _Allocator& __a, _Fp&& __f)
-             : __f_(allocator_arg, __a, _VSTD::forward<_Fp>(__f)),
-               __p_(allocator_arg, __a) {}
+             : __f_(allocator_arg_t(), __a, _VSTD::forward<_Fp>(__f)),
+               __p_(allocator_arg_t(), __a) {}
     // ~packaged_task() = default;
 
     // no copy
@@ -2021,8 +2021,8 @@
               class = __enable_if_t<!is_same<__remove_cvref_t<_Fp>, packaged_task>::value> >
         _LIBCPP_INLINE_VISIBILITY
         packaged_task(allocator_arg_t, const _Allocator& __a, _Fp&& __f)
-             : __f_(allocator_arg, __a, _VSTD::forward<_Fp>(__f)),
-               __p_(allocator_arg, __a) {}
+             : __f_(allocator_arg_t(), __a, _VSTD::forward<_Fp>(__f)),
+               __p_(allocator_arg_t(), __a) {}
     // ~packaged_task() = default;
 
     // no copy
diff --git a/libcxx/include/mutex b/libcxx/include/mutex
--- a/libcxx/include/mutex
+++ b/libcxx/include/mutex
@@ -328,7 +328,7 @@
 _LIBCPP_HIDE_FROM_ABI int
 try_lock(_L0& __l0, _L1& __l1)
 {
-    unique_lock<_L0> __u0(__l0, try_to_lock);
+    unique_lock<_L0> __u0(__l0, try_to_lock_t());
     if (__u0.owns_lock())
     {
         if (__l1.try_lock())
diff --git a/libcxx/test/libcxx/thread/thread.mutex/thread_safety_annotations_not_enabled.pass.cpp b/libcxx/test/libcxx/thread/thread.mutex/thread_safety_annotations_not_enabled.pass.cpp
--- a/libcxx/test/libcxx/thread/thread.mutex/thread_safety_annotations_not_enabled.pass.cpp
+++ b/libcxx/test/libcxx/thread/thread.mutex/thread_safety_annotations_not_enabled.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03
 
 // <mutex>
 
diff --git a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.guard/adopt_lock.pass.cpp b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.guard/adopt_lock.pass.cpp
--- a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.guard/adopt_lock.pass.cpp
+++ b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.guard/adopt_lock.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03
 
 // <mutex>
 
diff --git a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_adopt_lock.pass.cpp b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_adopt_lock.pass.cpp
--- a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_adopt_lock.pass.cpp
+++ b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_adopt_lock.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03
 
 // <mutex>
 
diff --git a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_defer_lock.pass.cpp b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_defer_lock.pass.cpp
--- a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_defer_lock.pass.cpp
+++ b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_defer_lock.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03
 
 // <mutex>
 
diff --git a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_try_to_lock.pass.cpp b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_try_to_lock.pass.cpp
--- a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_try_to_lock.pass.cpp
+++ b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.cons/mutex_try_to_lock.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03
 // ALLOW_RETRIES: 2
 
 // <mutex>
diff --git a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/lock.pass.cpp b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/lock.pass.cpp
--- a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/lock.pass.cpp
+++ b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/lock.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03
 // ALLOW_RETRIES: 2
 
 // <mutex>
diff --git a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock.pass.cpp b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock.pass.cpp
--- a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock.pass.cpp
+++ b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock.pass.cpp
@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
-//
+// UNSUPPORTED: c++03
 // ALLOW_RETRIES: 2
 
 // <mutex>
diff --git a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock_for.pass.cpp b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock_for.pass.cpp
--- a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock_for.pass.cpp
+++ b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock_for.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03
 
 // <mutex>
 
diff --git a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock_until.pass.cpp b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock_until.pass.cpp
--- a/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock_until.pass.cpp
+++ b/libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.unique/thread.lock.unique.locking/try_lock_until.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03
 
 // <mutex>
 
diff --git a/libcxx/test/std/thread/thread.mutex/thread.lock/types.pass.cpp b/libcxx/test/std/thread/thread.mutex/thread.lock/types.pass.cpp
--- a/libcxx/test/std/thread/thread.mutex/thread.lock/types.pass.cpp
+++ b/libcxx/test/std/thread/thread.mutex/thread.lock/types.pass.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: no-threads
+// UNSUPPORTED: c++03
 
 // <mutex>
 
diff --git a/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.pass.cpp b/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.pass.cpp
--- a/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.pass.cpp
+++ b/libcxx/test/std/utilities/memory/allocator.tag/allocator_arg.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: c++03
+
 // <memory>
 
 // struct allocator_arg_t { explicit allocator_arg_t() = default; };