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
@@ -8,22 +8,46 @@
  */
 
 #if !defined(__STDDEF_H) || defined(__need_ptrdiff_t) ||                       \
-    defined(__need_size_t) || defined(__need_wchar_t) ||                       \
-    defined(__need_NULL) || defined(__need_wint_t)
+    defined(__need_size_t) || defined(__need_rsize_t) ||                       \
+    defined(__need_wchar_t) || defined(__need_NULL) ||                         \
+    defined(__need_nullptr_t) || defined(__need_unreachable) ||                \
+    defined(__need_max_align_t) || defined(__need_offsetof) ||                 \
+    defined(__need_wint_t) ||                                                  \
+    (defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1)
 
 #if !defined(__need_ptrdiff_t) && !defined(__need_size_t) &&                   \
-    !defined(__need_wchar_t) && !defined(__need_NULL) &&                       \
-    !defined(__need_wint_t)
+    !defined(__need_rsize_t) && !defined(__need_wchar_t) &&                    \
+    !defined(__need_NULL) && !defined(__need_nullptr_t) &&                     \
+    !defined(__need_unreachable) && !defined(__need_max_align_t) &&            \
+    !defined(__need_offsetof) && !defined(__need_wint_t)
 /* Always define miscellaneous pieces when modules are available. */
 #if !__has_feature(modules)
 #define __STDDEF_H
 #endif
 #define __need_ptrdiff_t
 #define __need_size_t
+/* ISO9899:2011 7.20 (C11 Annex K): Define rsize_t if __STDC_WANT_LIB_EXT1__ is
+ * enabled. */
+#if defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1
+#define __need_rsize_t
+#endif
 #define __need_wchar_t
 #define __need_NULL
-#define __need_STDDEF_H_misc
-/* __need_wint_t is intentionally not defined here. */
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L) ||              \
+    defined(__cplusplus)
+#define __need_nullptr_t
+#endif
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#define __need_unreachable
+#endif
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) ||              \
+    (defined(__cplusplus) && __cplusplus >= 201103L)
+#define __need_max_align_t
+#endif
+#define __need_offsetof
+/* wint_t is provided by <wchar.h> and not <stddef.h>. It's here
+ * for compatibility, but must be explicitly requested. Therefore
+ * __need_wint_t is intentionally not defined here. */
 #endif
 
 #if defined(__need_ptrdiff_t)
@@ -48,18 +72,16 @@
 #undef __need_size_t
 #endif /*defined(__need_size_t) */
 
-#if defined(__need_STDDEF_H_misc)
-/* ISO9899:2011 7.20 (C11 Annex K): Define rsize_t if __STDC_WANT_LIB_EXT1__ is
- * enabled. */
-#if (defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1 && \
-     !defined(_RSIZE_T)) || __has_feature(modules)
+#if defined(__need_rsize_t)
+#if !defined(_RSIZE_T) || __has_feature(modules)
 /* Always define rsize_t when modules are available. */
 #if !__has_feature(modules)
 #define _RSIZE_T
 #endif
 typedef __SIZE_TYPE__ rsize_t;
 #endif
-#endif /* defined(__need_STDDEF_H_misc) */
+#undef __need_rsize_t
+#endif /* defined(__need_rsize_t) */
 
 #if defined(__need_wchar_t)
 #if !defined(__cplusplus) || (defined(_MSC_VER) && !_NATIVE_WCHAR_T_DEFINED)
@@ -88,32 +110,37 @@
 #else
 #  define NULL ((void*)0)
 #endif
+#undef __need_NULL
+#endif /* defined(__need_NULL) */
+
+#if defined(__need_nullptr_t)
 #ifdef __cplusplus
 #if defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED)
-namespace std { typedef decltype(nullptr) nullptr_t; }
+namespace std {
+typedef decltype(nullptr) nullptr_t;
+}
 using ::std::nullptr_t;
 #endif
-#endif
-#undef __need_NULL
-#endif /* defined(__need_NULL) */
-
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#else
 typedef typeof(nullptr) nullptr_t;
-#endif /* defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L */
+#endif
+#undef __need_nullptr_t
+#endif /* defined(__need_nullptr_t) */
 
-#if defined(__need_STDDEF_H_misc) && defined(__STDC_VERSION__) &&              \
-    __STDC_VERSION__ >= 202311L
+#if defined(__need_unreachable)
 #define unreachable() __builtin_unreachable()
-#endif /* defined(__need_STDDEF_H_misc) && >= C23 */
+#undef __need_unreachable
+#endif /* defined(__need_unreachable) */
 
-#if defined(__need_STDDEF_H_misc)
-#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) ||              \
-    (defined(__cplusplus) && __cplusplus >= 201103L)
+#if defined(__need_max_align_t)
 #include "__stddef_max_align_t.h"
-#endif
+#undef __need_max_align_t
+#endif /* defined(__need_max_align_t) */
+
+#if defined(__need_offsetof)
 #define offsetof(t, d) __builtin_offsetof(t, d)
-#undef __need_STDDEF_H_misc
-#endif  /* defined(__need_STDDEF_H_misc) */
+#undef __need_offsetof
+#endif /* defined(__need_offsetof) */
 
 /* Some C libraries expect to see a wint_t here. Others (notably MinGW) will use
 __WINT_TYPE__ directly; accommodate both by requiring __need_wint_t */
diff --git a/clang/test/Headers/stddef.c b/clang/test/Headers/stddef.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Headers/stddef.c
@@ -0,0 +1,46 @@
+// RUN: %clang_cc1 -fsyntax-only -verify=c99 -std=c99 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c11 -std=c11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c23 -std=c23 %s
+
+struct astruct { char member; };
+
+ptrdiff_t p0; // c99-error{{unknown type name 'ptrdiff_t'}} c11-error{{unknown type}} c23-error{{unknown type}}
+size_t s0; // c99-error{{unknown type name 'size_t'}} c11-error{{unknown type}} c23-error{{unknown type}}
+rsize_t r0; // c99-error{{unknown type name 'rsize_t'}} c11-error{{unknown type}} c23-error{{unknown type}}
+wchar_t wc0; // c99-error{{unknown type name 'wchar_t'}} c11-error{{unknown type}} c23-error{{unknown type}}
+void *v0 = NULL; // c99-error{{use of undeclared identifier 'NULL'}} c11-error{{undeclared identifier}} c23-error{{undeclared identifier}}
+nullptr_t n0; // c99-error{{unknown type name 'nullptr_t'}} c11-error{{unknown type}} c23-error{{unknown type}}
+static void f0(void) { unreachable(); } // c99-error{{call to undeclared function 'unreachable'}} c11-error{{undeclared function}} c23-error{{undeclared identifier}}
+max_align_t m0; // c99-error{{unknown type name 'max_align_t'}} c11-error{{unknown type}} c23-error{{unknown type}}
+size_t o0 = offsetof(struct astruct, member); // c99-error{{unknown type name 'size_t'}} c99-error{{call to undeclared function 'offsetof'}} c99-error{{expected expression}} c99-error{{use of undeclared identifier 'member'}} \
+                                                 c11-error{{unknown type}} c11-error{{undeclared function}} c11-error{{expected expression}} c11-error{{undeclared identifier}} \
+                                                 c23-error{{unknown type}} c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}}
+wint_t wi0; // c99-error{{unknown type name 'wint_t'}} c11-error{{unknown type}} c23-error{{unknown type}}
+
+#include <stddef.h>
+
+ptrdiff_t p1;
+size_t s1;
+rsize_t r1; // c99-error{{unknown type}} c11-error{{unknown type}} c23-error{{unknown type}}
+            // c99-note@stddef.h:*{{'size_t' declared here}} c11-note@stddef.h:*{{'size_t' declared here}} c23-note@stddef.h:*{{'size_t' declared here}}
+wchar_t wc1;
+void *v1 = NULL;
+nullptr_t n1; // c99-error{{unknown type}} c11-error{{unknown type}}
+static void f1(void) { unreachable(); } // c99-error{{undeclared function}} c11-error{{undeclared function}}
+max_align_t m1; // c99-error{{unknown type}}
+size_t o1 = offsetof(struct astruct, member);
+wint_t wi1; // c99-error{{unknown type}} c11-error{{unknown type}} c23-error{{unknown type}}
+
+// rsize_t needs to be opted into via __STDC_WANT_LIB_EXT1__ >= 1.
+#define __STDC_WANT_LIB_EXT1__ 1
+#include <stddef.h>
+ptrdiff_t p2;
+size_t s2;
+rsize_t r2;
+wchar_t wc2;
+void *v2 = NULL;
+nullptr_t n2; // c99-error{{unknown type}} c11-error{{unknown type}}
+static void f2(void) { unreachable(); } // c99-error{{undeclared function}} c11-error{{undeclared function}}
+max_align_t m2; // c99-error{{unknown type}}
+size_t o2 = offsetof(struct astruct, member);
+wint_t wi2; // c99-error{{unknown type}} c11-error{{unknown type}} c23-error{{unknown type}}
diff --git a/clang/test/Headers/stddefneeds.c b/clang/test/Headers/stddefneeds.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Headers/stddefneeds.c
@@ -0,0 +1,169 @@
+// RUN: %clang_cc1 -fsyntax-only -verify=c99 -std=c99 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c23 -std=c23 %s
+
+// Use C99 to verify that __need_ can be used to get types that wouldn't normally be available.
+
+struct astruct { char member; };
+
+ptrdiff_t p0; // c99-error{{unknown type name 'ptrdiff_t'}} c23-error{{unknown type}}
+size_t s0; // c99-error{{unknown type name 'size_t'}} c23-error{{unknown type}}
+rsize_t r0; // c99-error{{unknown type name 'rsize_t'}} c23-error{{unknown type}}
+wchar_t wc0; // c99-error{{unknown type name 'wchar_t'}} c23-error{{unknown type}}
+void *v0 = NULL; // c99-error{{use of undeclared identifier 'NULL'}} c23-error{{undeclared identifier}}
+nullptr_t n0; // c99-error{{unknown type name 'nullptr_t'}} c23-error{{unknown type}}
+static void f0(void) { unreachable(); } // c99-error{{call to undeclared function 'unreachable'}} c23-error{{undeclared identifier 'unreachable'}}
+max_align_t m0; // c99-error{{unknown type name 'max_align_t'}} c23-error{{unknown type}}
+size_t o0 = offsetof(struct astruct, member); // c99-error{{unknown type name 'size_t'}} c99-error{{call to undeclared function 'offsetof'}} c99-error{{expected expression}} c99-error{{use of undeclared identifier 'member'}} \
+                                                 c23-error{{unknown type name 'size_t'}} c23-error{{undeclared identifier 'offsetof'}} c23-error{{expected expression}} c23-error{{use of undeclared identifier 'member'}}
+wint_t wi0; // c99-error{{unknown type name 'wint_t'}} c23-error{{unknown type}}
+
+#define __need_ptrdiff_t
+#include <stddef.h>
+
+ptrdiff_t p1;
+size_t s1; // c99-error{{unknown type}} c23-error{{unknown type}}
+rsize_t r1; // c99-error{{unknown type}} c23-error{{unknown type}}
+wchar_t wc1; // c99-error{{unknown type}} c23-error{{unknown type}}
+void *v1 = NULL; // c99-error{{undeclared identifier}} c23-error{{undeclared identifier}}
+nullptr_t n1; // c99-error{{unknown type}} c23-error{{unknown type}}
+static void f1(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}}
+max_align_t m1; // c99-error{{unknown type}} c23-error{{unknown type}}
+size_t o1 = offsetof(struct astruct, member); // c99-error{{unknown type}} c99-error{{expected expression}} c99-error{{undeclared identifier}} \
+                                                 c23-error{{unknown type}} c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}}
+wint_t wi1; // c99-error{{unknown type}} c23-error{{unknown type}}
+
+#define __need_size_t
+#include <stddef.h>
+
+ptrdiff_t p2;
+size_t s2;
+rsize_t r2; // c99-error{{unknown type}} c23-error{{unknown type}}
+            // c99-note@stddef.h:*{{'size_t' declared here}} c23-note@stddef.h:*{{'size_t' declared here}}
+wchar_t wc2; // c99-error{{unknown type}} c23-error{{unknown type}}
+void *v2 = NULL; // c99-error{{undeclared identifier}} c23-error{{undeclared identifier}}
+nullptr_t n2; // c99-error{{unknown type}} c23-error{{unknown type}}
+static void f2(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}}
+max_align_t m2; // c99-error{{unknown type}} c23-error{{unknown type}}
+size_t o2 = offsetof(struct astruct, member); // c99-error{{expected expression}} c99-error{{undeclared identifier}} \
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}}
+wint_t wi2; // c99-error{{unknown type}} c23-error{{unknown type}}
+
+#define __need_rsize_t
+#include <stddef.h>
+
+ptrdiff_t p3;
+size_t s3;
+rsize_t r3;
+wchar_t wc3; // c99-error{{unknown type}} c23-error{{unknown type}}
+void *v3 = NULL; // c99-error{{undeclared identifier}} c23-error{{undeclared identifier}}
+nullptr_t n3; // c99-error{{unknown type}} c23-error{{unknown type}}
+static void f3(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}}
+max_align_t m3; // c99-error{{unknown type}} c23-error{{unknown type}}
+size_t o3 = offsetof(struct astruct, member); // c99-error{{expected expression}} c99-error{{undeclared identifier}} \
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}}
+wint_t wi3; // c99-error{{unknown type}} c23-error{{unknown type}}
+
+#define __need_wchar_t
+#include <stddef.h>
+
+ptrdiff_t p4;
+size_t s4;
+rsize_t r4;
+wchar_t wc4;
+void *v4 = NULL; // c99-error{{undeclared identifier}} c23-error{{undeclared identifier}}
+nullptr_t n4; // c99-error{{unknown type}} c23-error{{unknown type}}
+static void f4(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}}
+max_align_t m4; // c99-error{{unknown type}} c23-error{{unknown type}}
+size_t o4 = offsetof(struct astruct, member); // c99-error{{expected expression}} c99-error{{undeclared identifier}} \
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}}
+wint_t wi4; // c99-error{{unknown type}} c23-error{{unknown type}}
+
+#define __need_NULL
+#include <stddef.h>
+
+ptrdiff_t p5;
+size_t s5;
+rsize_t r5;
+wchar_t wc5;
+void *v5 = NULL;
+nullptr_t n5; // c99-error{{unknown type}} c23-error{{unknown type}}
+static void f5(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}}
+max_align_t m5; // c99-error{{unknown type}} c23-error{{unknown type}}
+size_t o5 = offsetof(struct astruct, member); // c99-error{{expected expression}} c99-error{{undeclared identifier}} \
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}}
+wint_t wi5; // c99-error{{unknown type}} c23-error{{unknown type}}
+
+// __need_nullptr_t generates an error in <C23 because its definition
+// depends on nullptr.
+#define __need_nullptr_t
+#include <stddef.h> // c99-error@stddef.h:*{{expected function body}}
+
+ptrdiff_t p6;
+size_t s6;
+rsize_t r6;
+wchar_t wc6;
+void *v6 = NULL;
+nullptr_t n6; // c99-error{{unknown type}}
+static void f6(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}}
+max_align_t m6; // c99-error{{unknown type}} c23-error{{unknown type}}
+size_t o6 = offsetof(struct astruct, member); // c99-error{{expected expression}} c99-error{{undeclared identifier}} \
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}}
+wint_t wi6; // c99-error{{unknown type}} c23-error{{unknown type}}
+
+#define __need_unreachable
+#include <stddef.h>
+
+ptrdiff_t p7;
+size_t s7;
+rsize_t r7;
+wchar_t wc7;
+void *v7 = NULL;
+nullptr_t n7 ; // c99-error{{unknown type}}
+static void f7(void) { unreachable(); }
+max_align_t m7; // c99-error{{unknown type}} c23-error{{unknown type}}
+size_t o7 = offsetof(struct astruct, member); // c99-error{{expected expression}} c99-error{{undeclared identifier}} \
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}}
+wint_t wi7; // c99-error{{unknown type}} c23-error{{unknown type}}
+
+#define __need_max_align_t
+#include <stddef.h>
+
+ptrdiff_t p8;
+size_t s8;
+rsize_t r8;
+wchar_t wc8;
+void *v8 = NULL;
+nullptr_t n8; // c99-error{{unknown type}}
+static void f8(void) { unreachable(); }
+max_align_t m8;
+size_t o8 = offsetof(struct astruct, member); // c99-error{{expected expression}} c99-error{{undeclared identifier}} \
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}}
+wint_t wi8; // c99-error{{unknown type}} c23-error{{unknown type}}
+
+#define __need_offsetof
+#include <stddef.h>
+
+ptrdiff_t p9;
+size_t s9;
+rsize_t r9;
+nullptr_t n9; // c99-error{{unknown type}}
+static void f9(void) { unreachable(); }
+wchar_t wc9;
+void *v9 = NULL;
+max_align_t m9;
+size_t o9 = offsetof(struct astruct, member);
+wint_t wi9; // c99-error{{unknown type}} c23-error{{unknown type}}
+
+#define __need_wint_t
+#include <stddef.h>
+
+ptrdiff_t p10;
+size_t s10;
+rsize_t r10;
+wchar_t wc10;
+void *v10 = NULL;
+nullptr_t n10; // c99-error{{unknown type}}
+static void f10(void) { unreachable(); }
+max_align_t m10;
+size_t o10 = offsetof(struct astruct, member);
+wint_t wi10;