diff --git a/clang/lib/Headers/CMakeLists.txt b/clang/lib/Headers/CMakeLists.txt
--- a/clang/lib/Headers/CMakeLists.txt
+++ b/clang/lib/Headers/CMakeLists.txt
@@ -14,6 +14,7 @@
   stdbool.h
   stddef.h
   __stddef_max_align_t.h
+  __stddef_null.h
   stdint.h
   stdnoreturn.h
   tgmath.h
diff --git a/clang/lib/Headers/__stddef_null.h b/clang/lib/Headers/__stddef_null.h
new file mode 100644
--- /dev/null
+++ b/clang/lib/Headers/__stddef_null.h
@@ -0,0 +1,32 @@
+/*===---- __stddef_null.h - Definition of NULL -----------------------------===
+ *
+ * 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
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#ifndef __STDDEF_NULL_H
+#define __STDDEF_NULL_H
+
+#undef NULL
+#ifdef __cplusplus
+#if !defined(__MINGW32__) && !defined(_MSC_VER)
+#define NULL __null
+#else
+#define NULL 0
+#endif
+#else
+#define NULL ((void *)0)
+#endif
+#ifdef __cplusplus
+#if defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED)
+namespace std {
+typedef decltype(nullptr) nullptr_t;
+}
+using ::std::nullptr_t;
+#endif
+#endif
+
+#endif
diff --git a/clang/lib/Headers/stddef.h b/clang/lib/Headers/stddef.h
--- a/clang/lib/Headers/stddef.h
+++ b/clang/lib/Headers/stddef.h
@@ -78,22 +78,7 @@
 #endif /* defined(__need_wchar_t) */
 
 #if defined(__need_NULL)
-#undef NULL
-#ifdef __cplusplus
-#  if !defined(__MINGW32__) && !defined(_MSC_VER)
-#    define NULL __null
-#  else
-#    define NULL 0
-#  endif
-#else
-#  define NULL ((void*)0)
-#endif
-#ifdef __cplusplus
-#if defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED)
-namespace std { typedef decltype(nullptr) nullptr_t; }
-using ::std::nullptr_t;
-#endif
-#endif
+#include <__stddef_null.h>
 #undef __need_NULL
 #endif /* defined(__need_NULL) */