diff --git a/libcxx/include/ext/__hash b/libcxx/include/ext/__hash
--- a/libcxx/include/ext/__hash
+++ b/libcxx/include/ext/__hash
@@ -13,6 +13,7 @@
 #  pragma GCC system_header
 
 #include <__config>
+#include <cstddef>
 #include <cstring>
 #include <string>
 
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -75,14 +75,12 @@
       export Darwin.C.stdint
     }
     module stdio_h {
-      // <stdio.h>'s __need_* macros require textual inclusion.
-      textual header "stdio.h"
+      header "stdio.h"
       export *
       export Darwin.C.stdio
     }
     module stdlib_h {
-      // <stdlib.h>'s __need_* macros require textual inclusion.
-      textual header "stdlib.h"
+      header "stdlib.h"
       export *
     }
     module string_h {
@@ -96,8 +94,7 @@
     // <time.h> provided by C library.
     module wchar_h {
       @requires_LIBCXX_ENABLE_WIDE_CHARACTERS@
-      // <wchar.h>'s __need_* macros require textual inclusion.
-      textual header "wchar.h"
+      header "wchar.h"
       export *
     }
     module wctype_h {
diff --git a/libcxx/include/stdbool.h b/libcxx/include/stdbool.h
--- a/libcxx/include/stdbool.h
+++ b/libcxx/include/stdbool.h
@@ -6,6 +6,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+
 #ifndef _LIBCPP_STDBOOL_H
 #define _LIBCPP_STDBOOL_H
 
diff --git a/libcxx/include/stddef.h b/libcxx/include/stddef.h
--- a/libcxx/include/stddef.h
+++ b/libcxx/include/stddef.h
@@ -7,16 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if defined(__need_ptrdiff_t) || defined(__need_size_t) || \
-    defined(__need_wchar_t) || defined(__need_NULL) || defined(__need_wint_t)
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
-
-#include_next <stddef.h>
-
-#elif !defined(_LIBCPP_STDDEF_H)
+#ifndef _LIBCPP_STDDEF_H
 #define _LIBCPP_STDDEF_H
 
 /*
@@ -42,9 +33,38 @@
 #  pragma GCC system_header
 #endif
 
-#  if __has_include_next(<stddef.h>)
-#    include_next <stddef.h>
+#if __has_include_next(<stddef.h>)
+    // The Clang builtin headers only define the types we need when we request it explicitly.
+    // TODO: We should fix that in Clang and drop these defines.
+#  ifndef __need_ptrdiff_t
+#    define __need_ptrdiff_t
+#  endif
+#  ifndef __need_size_t
+#    define __need_size_t
 #  endif
+#  ifndef __need_wchar_t
+#    define __need_wchar_t
+#  endif
+#  ifndef __need_NULL
+#    define __need_NULL
+#  endif
+#  ifndef __need_STDDEF_H_misc
+#    define __need_STDDEF_H_misc
+#  endif
+
+#  include_next <stddef.h>
+
+    // Now re-include the header without requesting any specific types, so as to get
+    // any other remaining types from stddef.h. This can all go away once the Clang
+    // buitin headers stop using these macros.
+#  undef __need_ptrdiff_t
+#  undef __need_size_t
+#  undef __need_wchar_t
+#  undef __need_NULL
+#  undef __need_STDDEF_H_misc
+
+#  include_next <stddef.h>
+#endif
 
 #ifdef __cplusplus
     typedef decltype(nullptr) nullptr_t;
diff --git a/libcxx/include/stdio.h b/libcxx/include/stdio.h
--- a/libcxx/include/stdio.h
+++ b/libcxx/include/stdio.h
@@ -7,15 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if defined(__need_FILE) || defined(__need___FILE)
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
-
-#include_next <stdio.h>
-
-#elif !defined(_LIBCPP_STDIO_H)
+#ifndef _LIBCPP_STDIO_H
 #define _LIBCPP_STDIO_H
 
 /*
diff --git a/libcxx/include/stdlib.h b/libcxx/include/stdlib.h
--- a/libcxx/include/stdlib.h
+++ b/libcxx/include/stdlib.h
@@ -7,15 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if defined(__need_malloc_and_calloc)
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
-
-#include_next <stdlib.h>
-
-#elif !defined(_LIBCPP_STDLIB_H)
+#ifndef _LIBCPP_STDLIB_H
 #define _LIBCPP_STDLIB_H
 
 /*
diff --git a/libcxx/include/wchar.h b/libcxx/include/wchar.h
--- a/libcxx/include/wchar.h
+++ b/libcxx/include/wchar.h
@@ -7,15 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if defined(__need_wint_t) || defined(__need_mbstate_t)
-
-#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#  pragma GCC system_header
-#endif
-
-#include_next <wchar.h>
-
-#elif !defined(_LIBCPP_WCHAR_H)
+#ifndef _LIBCPP_WCHAR_H
 #define _LIBCPP_WCHAR_H
 
 /*