diff --git a/libcxx/include/__hash_table b/libcxx/include/__hash_table
index 3a2de1d52dbd..71cf5fbf00bf 100644
--- a/libcxx/include/__hash_table
+++ b/libcxx/include/__hash_table
@@ -19,6 +19,7 @@
 #include <cmath>
 
 #include <__undef_min_max>
+#include <__undef___deallocate>
 
 #include <__debug>
 
diff --git a/libcxx/include/__sso_allocator b/libcxx/include/__sso_allocator
index 645f2ba17459..ca3b937c0138 100644
--- a/libcxx/include/__sso_allocator
+++ b/libcxx/include/__sso_allocator
@@ -15,6 +15,8 @@
 #include <type_traits>
 #include <new>
 
+#include <__undef___deallocate>
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
diff --git a/libcxx/include/__undef___deallocate b/libcxx/include/__undef___deallocate
new file mode 100644
index 000000000000..2b4ad99dad38
--- /dev/null
+++ b/libcxx/include/__undef___deallocate
@@ -0,0 +1,18 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifdef __deallocate
+#if defined(_MSC_VER) && !defined(__clang__)
+_LIBCPP_WARNING("macro __deallocate is incompatible with C++.  #undefining __deallocate")
+#else
+#warning: macro __deallocate is incompatible with C++.  #undefining __deallocate
+#endif
+#undef __deallocate
+#endif
diff --git a/libcxx/include/experimental/dynarray b/libcxx/include/experimental/dynarray
index 0bc8dfe2cb7b..a0258628dfa6 100644
--- a/libcxx/include/experimental/dynarray
+++ b/libcxx/include/experimental/dynarray
@@ -104,6 +104,8 @@ public:
 #include <new>
 #include <algorithm>
 
+#include <__undef___deallocate>
+
 #if defined(_LIBCPP_NO_EXCEPTIONS)
     #include <cassert>
 #endif
diff --git a/libcxx/include/memory b/libcxx/include/memory
index 40fc8ae07eea..7085cedadaf9 100644
--- a/libcxx/include/memory
+++ b/libcxx/include/memory
@@ -615,6 +615,7 @@ void* align(size_t alignment, size_t size, void*& ptr, size_t& space);
 #endif
 
 #include <__undef_min_max>
+#include <__undef___deallocate>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
index 6aeb23f028bf..3c0700eca443 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -469,4 +469,5 @@ module std [system] {
   module __tree { header "__tree" export * }
   module __tuple { header "__tuple" export * }
   module __undef_min_max { header "__undef_min_max" export * }
+  module __undef___deallocate { header "__undef___deallocate" export * }
 }
diff --git a/libcxx/include/new b/libcxx/include/new
index a710ed93fb43..eebe5af30ed9 100644
--- a/libcxx/include/new
+++ b/libcxx/include/new
@@ -68,6 +68,8 @@ void  operator delete[](void* ptr, void*) noexcept;
 #include <exception>
 #include <cstddef>
 
+#include <__undef___deallocate>
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
diff --git a/libcxx/include/valarray b/libcxx/include/valarray
index 2b942046db9d..bdaa58836f39 100644
--- a/libcxx/include/valarray
+++ b/libcxx/include/valarray
@@ -348,6 +348,7 @@ template <class T> unspecified2 end(const valarray<T>& v);
 #include <new>
 
 #include <__undef_min_max>
+#include <__undef___deallocate>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header