diff --git a/clang/lib/Headers/__stddef_max_align_t.h b/clang/lib/Headers/__stddef_max_align_t.h
--- a/clang/lib/Headers/__stddef_max_align_t.h
+++ b/clang/lib/Headers/__stddef_max_align_t.h
@@ -1,4 +1,4 @@
-/*===---- __stddef_max_align_t.h - Definition of max_align_t for modules ---===
+/*===---- __stddef_max_align_t.h - Definition of max_align_t ---------------===
  *
  * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  * See https://llvm.org/LICENSE.txt for license information.
diff --git a/clang/lib/Headers/__stddef_null.h b/clang/lib/Headers/__stddef_null.h
--- a/clang/lib/Headers/__stddef_null.h
+++ b/clang/lib/Headers/__stddef_null.h
@@ -7,7 +7,15 @@
  *===-----------------------------------------------------------------------===
  */
 
+#if !defined(NULL) || !__has_feature(modules)
+
+/* linux/stddef.h will define NULL to 0. glibc (and other) headers then define
+ * __need_NULL and rely on stddef.h to redefine NULL to the correct value again.
+ * Modules don't support redefining macros like that, but support that pattern
+ * in the non-modules case.
+ */
 #undef NULL
+
 #ifdef __cplusplus
 #if !defined(__MINGW32__) && !defined(_MSC_VER)
 #define NULL __null
@@ -17,3 +25,5 @@
 #else
 #define NULL ((void *)0)
 #endif
+
+#endif
diff --git a/clang/lib/Headers/__stddef_nullptr_t.h b/clang/lib/Headers/__stddef_nullptr_t.h
--- a/clang/lib/Headers/__stddef_nullptr_t.h
+++ b/clang/lib/Headers/__stddef_nullptr_t.h
@@ -7,11 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_NULLPTR_T) || __has_feature(modules)
-/* Always define nullptr_t when modules are available. */
-#if !__has_feature(modules)
+#ifndef _NULLPTR_T
 #define _NULLPTR_T
-#endif
+
 #ifdef __cplusplus
 #if defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED)
 namespace std {
@@ -22,4 +20,5 @@
 #else
 typedef typeof(nullptr) nullptr_t;
 #endif
+
 #endif
diff --git a/clang/lib/Headers/__stddef_offsetof.h b/clang/lib/Headers/__stddef_offsetof.h
--- a/clang/lib/Headers/__stddef_offsetof.h
+++ b/clang/lib/Headers/__stddef_offsetof.h
@@ -7,7 +7,6 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(offsetof) || __has_feature(modules)
-/* Always define offsetof when modules are available. */
+#ifndef offsetof
 #define offsetof(t, d) __builtin_offsetof(t, d)
 #endif
diff --git a/clang/lib/Headers/__stddef_ptrdiff_t.h b/clang/lib/Headers/__stddef_ptrdiff_t.h
--- a/clang/lib/Headers/__stddef_ptrdiff_t.h
+++ b/clang/lib/Headers/__stddef_ptrdiff_t.h
@@ -7,10 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_PTRDIFF_T) || __has_feature(modules)
-/* Always define ptrdiff_t when modules are available. */
-#if !__has_feature(modules)
+#ifndef _PTRDIFF_T
 #define _PTRDIFF_T
-#endif
+
 typedef __PTRDIFF_TYPE__ ptrdiff_t;
+
 #endif
diff --git a/clang/lib/Headers/__stddef_rsize_t.h b/clang/lib/Headers/__stddef_rsize_t.h
--- a/clang/lib/Headers/__stddef_rsize_t.h
+++ b/clang/lib/Headers/__stddef_rsize_t.h
@@ -7,10 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_RSIZE_T) || __has_feature(modules)
-/* Always define rsize_t when modules are available. */
-#if !__has_feature(modules)
+#ifndef _RSIZE_T
 #define _RSIZE_T
-#endif
+
 typedef __SIZE_TYPE__ rsize_t;
+
 #endif
diff --git a/clang/lib/Headers/__stddef_size_t.h b/clang/lib/Headers/__stddef_size_t.h
--- a/clang/lib/Headers/__stddef_size_t.h
+++ b/clang/lib/Headers/__stddef_size_t.h
@@ -7,10 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(_SIZE_T) || __has_feature(modules)
-/* Always define size_t when modules are available. */
-#if !__has_feature(modules)
+#ifndef _SIZE_T
 #define _SIZE_T
-#endif
+
 typedef __SIZE_TYPE__ size_t;
+
 #endif
diff --git a/clang/lib/Headers/__stddef_unreachable.h b/clang/lib/Headers/__stddef_unreachable.h
--- a/clang/lib/Headers/__stddef_unreachable.h
+++ b/clang/lib/Headers/__stddef_unreachable.h
@@ -7,7 +7,6 @@
  *===-----------------------------------------------------------------------===
  */
 
-#if !defined(unreachable) || __has_feature(modules)
-/* Always define unreachable when modules are available. */
+#ifndef unreachable
 #define unreachable() __builtin_unreachable()
 #endif
diff --git a/clang/lib/Headers/__stddef_wchar_t.h b/clang/lib/Headers/__stddef_wchar_t.h
--- a/clang/lib/Headers/__stddef_wchar_t.h
+++ b/clang/lib/Headers/__stddef_wchar_t.h
@@ -8,14 +8,16 @@
  */
 
 #if !defined(__cplusplus) || (defined(_MSC_VER) && !_NATIVE_WCHAR_T_DEFINED)
-/* Always define wchar_t when modules are available. */
-#if !defined(_WCHAR_T) || __has_feature(modules)
-#if !__has_feature(modules)
+
+#ifndef _WCHAR_T
 #define _WCHAR_T
-#if defined(_MSC_EXTENSIONS)
+
+#ifdef _MSC_EXTENSIONS
 #define _WCHAR_T_DEFINED
 #endif
-#endif
+
 typedef __WCHAR_TYPE__ wchar_t;
+
 #endif
+
 #endif
diff --git a/clang/lib/Headers/__stddef_wint_t.h b/clang/lib/Headers/__stddef_wint_t.h
--- a/clang/lib/Headers/__stddef_wint_t.h
+++ b/clang/lib/Headers/__stddef_wint_t.h
@@ -7,10 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
-/* Always define wint_t when modules are available. */
-#if !defined(_WINT_T) || __has_feature(modules)
-#if !__has_feature(modules)
+#ifndef _WINT_T
 #define _WINT_T
-#endif
+
 typedef __WINT_TYPE__ wint_t;
+
 #endif
diff --git a/clang/lib/Headers/module.modulemap b/clang/lib/Headers/module.modulemap
--- a/clang/lib/Headers/module.modulemap
+++ b/clang/lib/Headers/module.modulemap
@@ -153,8 +153,143 @@
   }
 }
 
-module _Builtin_stddef_max_align_t [system] [extern_c] {
-  header "__stddef_max_align_t.h"
+module _Builtin_float [system] {
+  header "float.h"
+  export *
+}
+
+module _Builtin_inttypes [system] {
+  header "inttypes.h"
+  export *
+}
+
+module _Builtin_iso646 [system] {
+  header "iso646.h"
+  export *
+}
+
+module _Builtin_limits [system] {
+  header "limits.h"
+  export *
+}
+
+module _Builtin_stdalign [system] {
+  header "stdalign.h"
+  export *
+}
+
+module _Builtin_stdarg [system] {
+  textual header "stdarg.h"
+
+  explicit module __gnuc_va_list {
+    header "__stdarg___gnuc_va_list.h"
+    export *
+  }
+
+  explicit module __va_copy {
+    header "__stdarg___va_copy.h"
+    export *
+  }
+
+  explicit module va_arg {
+    header "__stdarg_va_arg.h"
+    export *
+  }
+
+  explicit module va_copy {
+    header "__stdarg_va_copy.h"
+    export *
+  }
+
+  explicit module va_list {
+    header "__stdarg_va_list.h"
+    export *
+  }
+}
+
+module _Builtin_stdatomic [system] {
+  header "stdatomic.h"
+  export *
+}
+
+module _Builtin_stdbool [system] {
+  header "stdbool.h"
+  export *
+}
+
+module _Builtin_stddef [system] {
+  textual header "stddef.h"
+
+  explicit module max_align_t {
+    header "__stddef_max_align_t.h"
+    export *
+  }
+
+  explicit module null {
+    header "__stddef_null.h"
+    export *
+  }
+
+  explicit module nullptr_t {
+    requires c23
+    header "__stddef_nullptr_t.h"
+    export *
+  }
+
+  explicit module offsetof {
+    header "__stddef_offsetof.h"
+    export *
+  }
+
+  explicit module ptrdiff_t {
+    header "__stddef_ptrdiff_t.h"
+    export *
+  }
+
+  explicit module rsize_t {
+    header "__stddef_rsize_t.h"
+    export *
+  }
+
+  explicit module size_t {
+    header "__stddef_size_t.h"
+    export *
+  }
+
+  explicit module unreachable {
+    header "__stddef_unreachable.h"
+    export *
+  }
+
+  explicit module wchar_t {
+    header "__stddef_wchar_t.h"
+    export *
+  }
+
+  explicit module wint_t {
+    header "__stddef_wint_t.h"
+    export *
+  }
+}
+
+module _Builtin_stdint [system] {
+  header "stdint.h"
+  export *
+}
+
+module _Builtin_stdnoreturn [system] {
+  header "stdnoreturn.h"
+  export *
+}
+
+module _Builtin_tgmath [system] {
+  header "tgmath.h"
+  export *
+}
+
+module _Builtin_unwind [system] {
+  header "unwind.h"
+  export *
 }
 
 module opencl_c {
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
@@ -20,10 +20,7 @@
     !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
diff --git a/clang/test/Headers/stdarg.c b/clang/test/Headers/stdarg.c
--- a/clang/test/Headers/stdarg.c
+++ b/clang/test/Headers/stdarg.c
@@ -1,29 +1,46 @@
 // RUN: split-file %s %t
 // RUN: %clang_cc1 -fsyntax-only -verify=c89 -Werror=implicit-function-declaration -std=c89 %t/stdarg0.c
 // RUN: %clang_cc1 -fsyntax-only -verify=c99 -Werror=implicit-function-declaration -std=c99 %t/stdarg0.c
+// RUN: %clang_cc1 -fsyntax-only -verify=c89-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Werror=implicit-function-declaration -std=c89 %t/stdarg0.c
+// RUN: %clang_cc1 -fsyntax-only -verify=c99-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Werror=implicit-function-declaration -std=c99 %t/stdarg0.c
 // RUN: %clang_cc1 -fsyntax-only -verify=c89 -Werror=implicit-function-declaration -std=c89 %t/stdarg1.c
 // RUN: %clang_cc1 -fsyntax-only -verify=c99 -Werror=implicit-function-declaration -std=c99 %t/stdarg1.c
+// RUN: %clang_cc1 -fsyntax-only -verify=c89-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Werror=implicit-function-declaration -std=c89 %t/stdarg1.c
+// RUN: %clang_cc1 -fsyntax-only -verify=c99-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Werror=implicit-function-declaration -std=c99 %t/stdarg1.c
 
 // Split the file so that the "implicitly declaring library function" errors get repeated.
 
 //--- stdarg0.c
 static void f(int p, ...) {
-    __gnuc_va_list g; // c89-error{{undeclared identifier '__gnuc_va_list'}} c99-error{{undeclared identifier}}
-    va_list v; // c89-error{{undeclared identifier 'va_list'}} c99-error{{undeclared identifier}}
+    __gnuc_va_list g; // c89-error{{undeclared identifier '__gnuc_va_list'}} c99-error{{undeclared identifier}} \
+                         c89-modules-error{{undeclared identifier}} c99-modules-error{{undeclared identifier}}
+    va_list v; // c89-error{{undeclared identifier 'va_list'}} c99-error{{undeclared identifier}} \
+                  c89-modules-error{{undeclared identifier}} c99-modules-error{{undeclared identifier}}
     va_start(v, p); // c89-error{{implicitly declaring library function 'va_start'}} c89-note{{include the header <stdarg.h> or explicitly provide a declaration for 'va_start'}} c89-error{{undeclared identifier 'v'}} \
-                       c99-error{{call to undeclared library function 'va_start'}} c99-note{{provide a declaration}} c99-error{{undeclared identifier}}
+                       c99-error{{call to undeclared library function 'va_start'}} c99-note{{provide a declaration}} c99-error{{undeclared identifier}} \
+                       c89-modules-error{{implicitly declaring library function}} c89-modules-note{{provide a declaration}} c89-modules-error{{undeclared identifier}} \
+                       c99-modules-error{{undeclared library function}} c99-modules-note{{provide a declaration}} c99-modules-error{{undeclared identifier}}
     int i = va_arg(v, int); // c89-error{{implicit declaration of function 'va_arg'}} c89-error{{expected expression}} c89-error{{use of undeclared identifier 'v'}} \
-                               c99-error{{call to undeclared function 'va_arg'}} c99-error{{expected expression}} c99-error{{undeclared identifier}}
+                               c99-error{{call to undeclared function 'va_arg'}} c99-error{{expected expression}} c99-error{{undeclared identifier}} \
+                               c89-modules-error{{implicit declaration of function}} c89-modules-error{{expected expression}} c89-modules-error{{undeclared identifier}} \
+                               c99-modules-error{{undeclared function}} c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}}
     va_end(v); // c89-error{{implicitly declaring library function 'va_end'}} c89-note{{include the header <stdarg.h> or explicitly provide a declaration for 'va_end'}} c89-error{{undeclared identifier 'v'}} \
-                  c99-error{{call to undeclared library function 'va_end'}} c99-note{{provide a declaration}} c99-error{{undeclared identifier}}
+                  c99-error{{call to undeclared library function 'va_end'}} c99-note{{provide a declaration}} c99-error{{undeclared identifier}} \
+                  c89-modules-error{{implicitly declaring library function}} c89-modules-note{{provide a declaration}} c89-modules-error{{undeclared identifier}} \
+                  c99-modules-error{{undeclared library function}} c99-modules-note{{provide a declaration}} c99-modules-error{{undeclared identifier}}
     __va_copy(g, v); // c89-error{{implicit declaration of function '__va_copy'}} c89-error{{use of undeclared identifier 'g'}} c89-error{{use of undeclared identifier 'v'}} \
-                        c99-error{{call to undeclared function '__va_copy'}} c99-error{{undeclared identifier}} c99-error{{undeclared identifier}}
+                        c99-error{{call to undeclared function '__va_copy'}} c99-error{{undeclared identifier}} c99-error{{undeclared identifier}} \
+                        c89-modules-error{{implicit declaration of function}} c89-modules-error{{undeclared identifier}} c89-modules-error{{undeclared identifier}} \
+                        c99-modules-error{{undeclared function}} c99-modules-error{{undeclared identifier}} c99-modules-error{{undeclared identifier}}
     va_copy(g, v); // c89-error{{implicitly declaring library function 'va_copy'}} c89-note{{include the header <stdarg.h> or explicitly provide a declaration for 'va_copy'}} c89-error{{use of undeclared identifier 'g'}} c89-error{{use of undeclared identifier 'v'}} \
-                      c99-error{{call to undeclared library function 'va_copy'}} c99-note{{provide a declaration}} c99-error{{undeclared identifier}} c99-error{{undeclared identifier}}
+                      c99-error{{call to undeclared library function 'va_copy'}} c99-note{{provide a declaration}} c99-error{{undeclared identifier}} c99-error{{undeclared identifier}} \
+                      c89-modules-error{{implicitly declaring library function}} c89-modules-note{{provide a declaration}} c89-modules-error{{undeclared identifier}} c89-modules-error{{undeclared identifier}} \
+                      c99-modules-error{{undeclared library function}} c99-modules-note{{provide a declaration}} c99-modules-error{{undeclared identifier}} c99-modules-error{{undeclared identifier}}
 }
 
 //--- stdarg1.c
 // c99-no-diagnostics
+// c99-modules-no-diagnostics
 
 #include <stdarg.h>
 static void f(int p, ...) {
@@ -33,5 +50,6 @@
     int i = va_arg(v, int);
     va_end(v);
     __va_copy(g, v);
-    va_copy(g, v); // c89-error{{implicitly declaring library function}} c89-note{{provide a declaration}}
+    va_copy(g, v); // c89-error{{implicitly declaring library function}} c89-note{{provide a declaration}} \
+                      c89-modules-error{{implicitly declaring library function}} c89-modules-note{{provide a declaration}}
 }
diff --git a/clang/test/Headers/stdargneeds.c b/clang/test/Headers/stdargneeds.c
--- a/clang/test/Headers/stdargneeds.c
+++ b/clang/test/Headers/stdargneeds.c
@@ -1,23 +1,34 @@
 // RUN: split-file %s %t
 // RUN: %clang_cc1 -fsyntax-only -verify -Werror=implicit-function-declaration -std=c89 %t/stdargneeds0.c
+// RUN: %clang_cc1 -fsyntax-only -verify=expected-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Werror=implicit-function-declaration -std=c89 %t/stdargneeds0.c
 // RUN: %clang_cc1 -fsyntax-only -verify -Werror=implicit-function-declaration -std=c89 %t/stdargneeds1.c
+// RUN: %clang_cc1 -fsyntax-only -verify=expected-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Werror=implicit-function-declaration -std=c89 %t/stdargneeds1.c
 // RUN: %clang_cc1 -fsyntax-only -verify -Werror=implicit-function-declaration -std=c89 %t/stdargneeds2.c
+// RUN: %clang_cc1 -fsyntax-only -verify=expected-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Werror=implicit-function-declaration -std=c89 %t/stdargneeds2.c
 // RUN: %clang_cc1 -fsyntax-only -verify -Werror=implicit-function-declaration -std=c89 %t/stdargneeds3.c
+// RUN: %clang_cc1 -fsyntax-only -verify=expected-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Werror=implicit-function-declaration -std=c89 %t/stdargneeds3.c
 // RUN: %clang_cc1 -fsyntax-only -verify -Werror=implicit-function-declaration -std=c89 %t/stdargneeds4.c
+// RUN: %clang_cc1 -fsyntax-only -verify=expected-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Werror=implicit-function-declaration -std=c89 %t/stdargneeds4.c
 // RUN: %clang_cc1 -fsyntax-only -verify -Werror=implicit-function-declaration -std=c89 %t/stdargneeds5.c
+// RUN: %clang_cc1 -fsyntax-only -verify=expected-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Werror=implicit-function-declaration -std=c89 %t/stdargneeds5.c
 
 // Split the file so that the "implicitly declaring library function" errors get repeated.
 // Use C89 to verify that __need_ can be used to get types that wouldn't normally be available.
 
 //--- stdargneeds0.c
 static void f(int p, ...) {
-    __gnuc_va_list g; // expected-error{{undeclared identifier '__gnuc_va_list'}}
-    va_list v; // expected-error{{undeclared identifier 'va_list'}}
-    va_start(v, p); // expected-error{{implicitly declaring library function 'va_start'}} expected-note{{include the header <stdarg.h> or explicitly provide a declaration for 'va_start'}} expected-error{{undeclared identifier 'v'}}
-    int i = va_arg(v, int); // expected-error{{implicit declaration of function 'va_arg'}} expected-error{{expected expression}} expected-error{{use of undeclared identifier 'v'}}
-    va_end(v); // expected-error{{implicitly declaring library function 'va_end'}} expected-note{{include the header <stdarg.h> or explicitly provide a declaration for 'va_end'}} expected-error{{undeclared identifier 'v'}}
-    __va_copy(g, v); // expected-error{{implicit declaration of function '__va_copy'}} expected-error{{use of undeclared identifier 'g'}} expected-error{{use of undeclared identifier 'v'}}
-    va_copy(g, v); // expected-error{{implicitly declaring library function 'va_copy'}} expected-note{{include the header <stdarg.h> or explicitly provide a declaration for 'va_copy'}} expected-error{{use of undeclared identifier 'g'}} expected-error{{use of undeclared identifier 'v'}}
+    __gnuc_va_list g; // expected-error{{undeclared identifier '__gnuc_va_list'}} expected-modules-error{{undeclared identifier}}
+    va_list v; // expected-error{{undeclared identifier 'va_list'}} expected-modules-error{{undeclared identifier}}
+    va_start(v, p); // expected-error{{implicitly declaring library function 'va_start'}} expected-note{{include the header <stdarg.h> or explicitly provide a declaration for 'va_start'}} expected-error{{undeclared identifier 'v'}} \
+                       expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}} expected-modules-error{{undeclared identifier}}
+    int i = va_arg(v, int); // expected-error{{implicit declaration of function 'va_arg'}} expected-error{{expected expression}} expected-error{{use of undeclared identifier 'v'}} \
+                               expected-modules-error{{implicit declaration of function}} expected-modules-error{{expected expression}} expected-modules-error{{undeclared identifier}}
+    va_end(v); // expected-error{{implicitly declaring library function 'va_end'}} expected-note{{include the header <stdarg.h> or explicitly provide a declaration for 'va_end'}} expected-error{{undeclared identifier 'v'}} \
+                  expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}} expected-modules-error{{undeclared identifier}}
+    __va_copy(g, v); // expected-error{{implicit declaration of function '__va_copy'}} expected-error{{use of undeclared identifier 'g'}} expected-error{{use of undeclared identifier 'v'}} \
+                        expected-modules-error{{implicit declaration of function}} expected-modules-error{{undeclared identifier}} expected-modules-error{{undeclared identifier}}
+    va_copy(g, v); // expected-error{{implicitly declaring library function 'va_copy'}} expected-note{{include the header <stdarg.h> or explicitly provide a declaration for 'va_copy'}} expected-error{{use of undeclared identifier 'g'}} expected-error{{use of undeclared identifier 'v'}} \
+                      expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}} expected-modules-error{{undeclared identifier}} expected-modules-error{{undeclared identifier}}
 }
 
 //--- stdargneeds1.c
@@ -25,25 +36,37 @@
 #include <stdarg.h>
 static void f(int p, ...) {
     __gnuc_va_list g;
-    va_list v; // expected-error{{undeclared identifier}}
-    va_start(v, p); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} expected-error{{undeclared identifier}}
-    int i = va_arg(v, int); // expected-error{{implicit declaration of function}} expected-error{{expected expression}} expected-error{{undeclared identifier}}
-    va_end(v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} expected-error{{undeclared identifier}}
-    __va_copy(g, v); // expected-error{{implicit declaration of function}} expected-error{{undeclared identifier}}
-    va_copy(g, v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} expected-error{{undeclared identifier}}
+    va_list v; // expected-error{{undeclared identifier}} \
+                  expected-modules-error{{'va_list' must be declared before it is used}} expected-modules-note@__stdarg_va_list.h:*{{declaration here is not visible}}
+    va_start(v, p); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} expected-error{{undeclared identifier}} \
+                       expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
+    int i = va_arg(v, int); // expected-error{{implicit declaration of function}} expected-error{{expected expression}} expected-error{{undeclared identifier}} \
+                               expected-modules-error{{implicit declaration of function}} expected-modules-error{{expected expression}}
+    va_end(v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} expected-error{{undeclared identifier}} \
+                  expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
+    __va_copy(g, v); // expected-error{{implicit declaration of function}} expected-error{{undeclared identifier}} \
+                        expected-modules-error{{implicit declaration of function}}
+    va_copy(g, v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} expected-error{{undeclared identifier}} \
+                      expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
 }
 
 //--- stdargneeds2.c
 #define __need_va_list
 #include <stdarg.h>
 static void f(int p, ...) {
-    __gnuc_va_list g; // expected-error{{undeclared identifier}}
+    __gnuc_va_list g; // expected-error{{undeclared identifier}} \
+                         expected-modules-error{{'__gnuc_va_list' must be declared before it is used}} expected-modules-note@__stdarg___gnuc_va_list.h:*{{declaration here is not visible}}
     va_list v;
-    va_start(v, p); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}}
-    int i = va_arg(v, int); // expected-error{{implicit declaration of function}} expected-error{{expected expression}}
-    va_end(v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}}
-    __va_copy(g, v); // expected-error{{implicit declaration of function}} expected-error{{undeclared identifier}}
-    va_copy(g, v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} expected-error{{undeclared identifier}}
+    va_start(v, p); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} \
+                       expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
+    int i = va_arg(v, int); // expected-error{{implicit declaration of function}} expected-error{{expected expression}} \
+                               expected-modules-error{{implicit declaration of function}} expected-modules-error{{expected expression}}
+    va_end(v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} \
+                  expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
+    __va_copy(g, v); // expected-error{{implicit declaration of function}} expected-error{{undeclared identifier}} \
+                        expected-modules-error{{implicit declaration of function}}
+    va_copy(g, v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} expected-error{{undeclared identifier}} \
+                      expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
 }
 
 //--- stdargneeds3.c
@@ -51,13 +74,16 @@
 #define __need_va_arg
 #include <stdarg.h>
 static void f(int p, ...) {
-    __gnuc_va_list g; // expected-error{{undeclared identifier}}
+    __gnuc_va_list g; // expected-error{{undeclared identifier}} \
+                         expected-modules-error{{'__gnuc_va_list' must be declared before it is used}} expected-modules-note@__stdarg___gnuc_va_list.h:*{{declaration here is not visible}}
     va_list v;
     va_start(v, p);
     int i = va_arg(v, int);
     va_end(v);
-    __va_copy(g, v); // expected-error{{implicit declaration of function}} expected-error{{undeclared identifier}}
-    va_copy(g, v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} expected-error{{undeclared identifier}}
+    __va_copy(g, v); // expected-error{{implicit declaration of function}} expected-error{{undeclared identifier}} \
+                        expected-modules-error{{implicit declaration of function}}
+    va_copy(g, v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} expected-error{{undeclared identifier}} \
+                      expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
 }
 
 //--- stdargneeds4.c
@@ -68,11 +94,15 @@
 static void f(int p, ...) {
     __gnuc_va_list g;
     va_list v;
-    va_start(v, p); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}}
-    int i = va_arg(v, int); // expected-error{{implicit declaration of function}} expected-error{{expected expression}}
-    va_end(v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}}
+    va_start(v, p); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} \
+                       expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
+    int i = va_arg(v, int); // expected-error{{implicit declaration of function}} expected-error{{expected expression}} \
+                               expected-modules-error{{implicit declaration of function}} expected-modules-error{{expected expression}}
+    va_end(v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} \
+                  expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
     __va_copy(g, v);
-    va_copy(g, v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}}
+    va_copy(g, v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} \
+                      expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
 }
 
 //--- stdargneeds5.c
@@ -83,9 +113,12 @@
 static void f(int p, ...) {
     __gnuc_va_list g;
     va_list v;
-    va_start(v, p); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}}
-    int i = va_arg(v, int); // expected-error{{implicit declaration of function}} expected-error{{expected expression}}
-    va_end(v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}}
-    __va_copy(g, v); // expected-error{{implicit declaration of function}}
+    va_start(v, p); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} \
+                       expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
+    int i = va_arg(v, int); // expected-error{{implicit declaration of function}} expected-error{{expected expression}} \
+                               expected-modules-error{{implicit declaration of function}} expected-modules-error{{expected expression}}
+    va_end(v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}} \
+                  expected-modules-error{{implicitly declaring library function}} expected-modules-note{{provide a declaration}}
+    __va_copy(g, v); // expected-error{{implicit declaration of function}} expected-modules-error{{implicit declaration of function}}
     va_copy(g, v);
 }
diff --git a/clang/test/Headers/stddef.c b/clang/test/Headers/stddef.c
--- a/clang/test/Headers/stddef.c
+++ b/clang/test/Headers/stddef.c
@@ -1,35 +1,56 @@
 // 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
+// RUN: %clang_cc1 -fsyntax-only -verify=c99-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -std=c99 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c11-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -std=c11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c23-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -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}}
+ptrdiff_t p0; // c99-error{{unknown type name 'ptrdiff_t'}} c11-error{{unknown type}} c23-error{{unknown type}} \
+                 c99-modules-error{{unknown type}} c11-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+size_t s0; // c99-error{{unknown type name 'size_t'}} c11-error{{unknown type}} c23-error{{unknown type}} \
+              c99-modules-error{{unknown type}} c11-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+rsize_t r0; // c99-error{{unknown type name 'rsize_t'}} c11-error{{unknown type}} c23-error{{unknown type}} \
+               c99-modules-error{{unknown type}} c11-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+wchar_t wc0; // c99-error{{unknown type name 'wchar_t'}} c11-error{{unknown type}} c23-error{{unknown type}} \
+                c99-modules-error{{unknown type}} c11-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+void *v0 = NULL; // c99-error{{use of undeclared identifier 'NULL'}} c11-error{{undeclared identifier}} c23-error{{undeclared identifier}} \
+                    c99-modules-error{{undeclared identifier}} c11-modules-error{{undeclared identifier}} c23-modules-error{{undeclared identifier}}
+nullptr_t n0; // c99-error{{unknown type name 'nullptr_t'}} c11-error{{unknown type}} c23-error{{unknown type}} \
+                 c99-modules-error{{unknown type}} c11-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+static void f0(void) { unreachable(); } // c99-error{{call to undeclared function 'unreachable'}} c11-error{{undeclared function}} c23-error{{undeclared identifier}} \
+                                           c99-modules-error{{undeclared function}} c11-modules-error{{undeclared function}} c23-modules-error{{undeclared identifier}}
+max_align_t m0; // c99-error{{unknown type name 'max_align_t'}} c11-error{{unknown type}} c23-error{{unknown type}} \
+                   c99-modules-error{{unknown type}} c11-modules-error{{unknown type}} c23-modules-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}}
+                                                 c23-error{{unknown type}} c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}} \
+                                                 c99-modules-error{{unknown type}} c99-modules-error{{undeclared function}} c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}} \
+                                                 c11-modules-error{{unknown type}} c11-modules-error{{undeclared function}} c11-modules-error{{expected expression}} c11-modules-error{{undeclared identifier}} \
+                                                 c23-modules-error{{unknown type}} c23-modules-error{{undeclared identifier}} c23-modules-error{{expected expression}} c23-modules-error{{undeclared identifier}}
+wint_t wi0; // c99-error{{unknown type name 'wint_t'}} c11-error{{unknown type}} c23-error{{unknown type}} \
+               c99-modules-error{{unknown type name 'wint_t'}} c11-modules-error{{unknown type}} c23-modules-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_size_t.h:*{{'size_t' declared here}} c11-note@__stddef_size_t.h:*{{'size_t' declared here}} c23-note@__stddef_size_t.h:*{{'size_t' declared here}}
+rsize_t r1; // c99-error{{unknown type}} c11-error{{unknown type}} c23-error{{unknown type}} \
+               c99-note@__stddef_size_t.h:*{{'size_t' declared here}} c11-note@__stddef_size_t.h:*{{'size_t' declared here}} c23-note@__stddef_size_t.h:*{{'size_t' declared here}} \
+               c99-modules-error{{'rsize_t' must be declared before it is used}} c11-modules-error{{must be declared}} c23-modules-error{{must be declared}} \
+               c99-modules-note@__stddef_rsize_t.h:*{{declaration here is not visible}} c11-modules-note@__stddef_rsize_t.h:*{{declaration here is not visible}} c23-modules-note@__stddef_rsize_t.h:*{{declaration here is not visible}}
 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}}
+nullptr_t n1; // c99-error{{unknown type}} c11-error{{unknown type}} \
+                 c99-modules-error{{unknown type}} c11-modules-error{{unknown type}}
+static void f1(void) { unreachable(); } // c99-error{{undeclared function}} c11-error{{undeclared function}} \
+                                           c99-modules-error{{undeclared function}} c11-modules-error{{undeclared function}}
+max_align_t m1; // c99-error{{unknown type}} c99-modules-error{{'max_align_t' must be declared before it is used}} \
+                   c99-modules-note@__stddef_max_align_t.h:*{{declaration here is not visible}}
 size_t o1 = offsetof(struct astruct, member);
-wint_t wi1; // c99-error{{unknown type}} c11-error{{unknown type}} c23-error{{unknown type}}
+wint_t wi1; // c99-error{{unknown type}} c11-error{{unknown type}} c23-error{{unknown type}} \
+               c99-modules-error{{'wint_t' must be declared before it is used}} c11-modules-error{{must be declared}} c23-modules-error{{must be declared}}
 
 // rsize_t needs to be opted into via __STDC_WANT_LIB_EXT1__ >= 1.
 #define __STDC_WANT_LIB_EXT1__ 1
@@ -39,8 +60,14 @@
 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}}
+nullptr_t n2; // c99-error{{unknown type}} c11-error{{unknown type}} \
+                 c99-modules-error{{unknown type}} c11-modules-error{{unknown type}}
+static void f2(void) { unreachable(); } // c99-error{{undeclared function}} c11-error{{undeclared function}} \
+                                           c99-modules-error{{undeclared function}} c11-modules-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}}
+wint_t wi2; // c99-error{{unknown type}} c11-error{{unknown type}} c23-error{{unknown type}} \
+               c99-modules-note@__stddef_wint_t.h:*{{declaration here is not visible}} c11-modules-note@__stddef_wint_t.h:*{{declaration here is not visible}} c23-modules-note@__stddef_wint_t.h:*{{declaration here is not visible}}
+
+// m2 and wi2 don't generate errors in modules, the "must be declared before used"
+// errors are only emitted the first time the known-but-not-visible type is seen.
diff --git a/clang/test/Headers/stddefneeds.c b/clang/test/Headers/stddefneeds.c
--- a/clang/test/Headers/stddefneeds.c
+++ b/clang/test/Headers/stddefneeds.c
@@ -1,36 +1,72 @@
 // RUN: %clang_cc1 -fsyntax-only -verify=c99 -std=c99 %s
 // RUN: %clang_cc1 -fsyntax-only -verify=c23 -std=c23 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c99-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -std=c99 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c23-modules -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -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}}
+ptrdiff_t p0; // c99-error{{unknown type name 'ptrdiff_t'}} c23-error{{unknown type}} \
+                 c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+size_t s0; // c99-error{{unknown type name 'size_t'}} c23-error{{unknown type}} \
+              c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+rsize_t r0; // c99-error{{unknown type name 'rsize_t'}} c23-error{{unknown type}} \
+               c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+wchar_t wc0; // c99-error{{unknown type name 'wchar_t'}} c23-error{{unknown type}} \
+                c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+void *v0 = NULL; // c99-error{{use of undeclared identifier 'NULL'}} c23-error{{undeclared identifier}} \
+                    c99-modules-error{{undeclared identifier}} c23-modules-error{{undeclared identifier}}
+nullptr_t n0; // c99-error{{unknown type name 'nullptr_t'}} c23-error{{unknown type}} \
+                 c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+static void f0(void) { unreachable(); } // c99-error{{call to undeclared function 'unreachable'}} c23-error{{undeclared identifier 'unreachable'}} \
+                                           c99-modules-error{{undeclared function}} c23-modules-error{{undeclared identifier}}
+max_align_t m0; // c99-error{{unknown type name 'max_align_t'}} c23-error{{unknown type}} \
+                   c99-modules-error{{unknown type}} c23-modules-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}}
+                                                 c23-error{{unknown type name 'size_t'}} c23-error{{undeclared identifier 'offsetof'}} c23-error{{expected expression}} c23-error{{use of undeclared identifier 'member'}} \
+                                                 c99-modules-error{{unknown type}} c99-modules-error{{undeclared function}} c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}} \
+                                                 c23-modules-error{{unknown type}} c23-modules-error{{undeclared identifier}} c23-modules-error{{expected expression}} c23-modules-error{{undeclared identifier}}
+wint_t wi0; // c99-error{{unknown type name 'wint_t'}} c23-error{{unknown type}} \
+               c99-modules-error{{unknown type}} c23-modules-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 s1; // c99-error{{unknown type}} c23-error{{unknown type}} \
+              c99-modules-error{{'size_t' must be declared before it is used}} c23-modules-error{{must be declared}} \
+              c99-modules-note@__stddef_size_t.h:*{{declaration here is not visible}} c23-modules-note@__stddef_size_t.h:*{{declaration here is not visible}}
+rsize_t r1; // c99-error{{unknown type}} c23-error{{unknown type}} \
+               c99-modules-error{{'rsize_t' must be declared before it is used}} c23-modules-error{{must be declared}} \
+               c99-modules-note@__stddef_rsize_t.h:*{{declaration here is not visible}} c23-modules-note@__stddef_rsize_t.h:*{{declaration here is not visible}}
+wchar_t wc1; // c99-error{{unknown type}} c23-error{{unknown type}} \
+                c99-modules-error{{'wchar_t' must be declared before it is used}} c23-modules-error{{must be declared}} \
+                c99-modules-note@__stddef_wchar_t.h:*{{declaration here is not visible}} c23-modules-note@__stddef_wchar_t.h:*{{declaration here is not visible}}
+void *v1 = NULL; // c99-error{{undeclared identifier}} c23-error{{undeclared identifier}} \
+                    c99-modules-error{{undeclared identifier}} c23-modules-error{{undeclared identifier}}
+nullptr_t n1; // c99-error{{unknown type}} c23-error{{unknown type}} \
+                 c99-modules-error{{unknown type}} c23-modules-error{{'nullptr_t' must be declared before it is used}} \
+                 c23-modules-note@__stddef_nullptr_t.h:*{{declaration here is not visible}}
+static void f1(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}} \
+                                           c99-modules-error{{undeclared function}} c23-modules-error{{undeclared identifier}}
+max_align_t m1; // c99-error{{unknown type}} c23-error{{unknown type}} \
+                   c99-modules-error{{'max_align_t' must be declared before it is used}} c23-modules-error{{must be declared}} \
+                   c99-modules-note@__stddef_max_align_t.h:*{{declaration here is not visible}} c23-modules-note@__stddef_max_align_t.h:*{{declaration here is not visible}}
 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}}
+                                                 c23-error{{unknown type}} c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}} \
+                                                 c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}} \
+                                                 c23-modules-error{{undeclared identifier}} c23-modules-error{{expected expression}} c23-modules-error{{undeclared identifier}}
+wint_t wi1; // c99-error{{unknown type}} c23-error{{unknown type}} \
+               c99-modules-error{{'wint_t' must be declared before it is used}} c23-modules-error{{must be declared}} \
+               c99-modules-note@__stddef_wint_t.h:*{{declaration here is not visible}} c23-modules-note@__stddef_wint_t.h:*{{declaration here is not visible}}
+
+// The "must be declared before used" errors are only emitted the first time a
+// known-but-not-visible type is seen. At this point the _Builtin_stddef module
+// has been built and all of the types tried, so most of the errors won't be
+// repeated below in modules. The types still aren't available, just the errors
+// aren't repeated. e.g. rsize_t still isn't available, if r1 above got deleted,
+// its error would move to r2 below.
 
 #define __need_size_t
 #include <stddef.h>
@@ -40,12 +76,17 @@
 rsize_t r2; // c99-error{{unknown type}} c23-error{{unknown type}}
             // c99-note@__stddef_size_t.h:*{{'size_t' declared here}} c23-note@__stddef_size_t.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}}
+void *v2 = NULL; // c99-error{{undeclared identifier}} c23-error{{undeclared identifier}} \
+                    c99-modules-error{{undeclared identifier}} c23-modules-error{{undeclared identifier}}
+nullptr_t n2; // c99-error{{unknown type}} c23-error{{unknown type}} \
+                 c99-modules-error{{unknown type}}
+static void f2(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}} \
+                                           c99-modules-error{{undeclared function}} c23-modules-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}}
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}} \
+                                                 c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}} \
+                                                 c23-modules-error{{undeclared identifier}} c23-modules-error{{expected expression}} c23-modules-error{{undeclared identifier}}
 wint_t wi2; // c99-error{{unknown type}} c23-error{{unknown type}}
 
 #define __need_rsize_t
@@ -55,12 +96,17 @@
 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}}
+void *v3 = NULL; // c99-error{{undeclared identifier}} c23-error{{undeclared identifier}} \
+                    c99-modules-error{{undeclared identifier}} c23-modules-error{{undeclared identifier}}
+nullptr_t n3; // c99-error{{unknown type}} c23-error{{unknown type}} \
+                 c99-modules-error{{unknown type}}
+static void f3(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}} \
+                                           c99-modules-error{{undeclared function}} c23-modules-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}}
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}} \
+                                                 c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}} \
+                                                 c23-modules-error{{undeclared identifier}} c23-modules-error{{expected expression}} c23-modules-error{{undeclared identifier}}
 wint_t wi3; // c99-error{{unknown type}} c23-error{{unknown type}}
 
 #define __need_wchar_t
@@ -70,12 +116,17 @@
 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}}
+void *v4 = NULL; // c99-error{{undeclared identifier}} c23-error{{undeclared identifier}} \
+                    c99-modules-error{{undeclared identifier}} c23-modules-error{{undeclared identifier}}
+nullptr_t n4; // c99-error{{unknown type}} c23-error{{unknown type}} \
+                 c99-modules-error{{unknown type}}
+static void f4(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}} \
+                                           c99-modules-error{{undeclared function}} c23-modules-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}}
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}} \
+                                                 c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}} \
+                                                 c23-modules-error{{undeclared identifier}} c23-modules-error{{expected expression}} c23-modules-error{{undeclared identifier}}
 wint_t wi4; // c99-error{{unknown type}} c23-error{{unknown type}}
 
 #define __need_NULL
@@ -86,28 +137,36 @@
 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}}
+nullptr_t n5; // c99-error{{unknown type}} c23-error{{unknown type}} \
+                 c99-modules-error{{unknown type}}
+static void f5(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}} \
+                                           c99-modules-error{{undeclared function}} c23-modules-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}}
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}} \
+                                                 c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}} \
+                                                 c23-modules-error{{undeclared identifier}} c23-modules-error{{expected expression}} c23-modules-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_nullptr_t.h:*{{expected function body}}
+#include <stddef.h> // c99-error@__stddef_nullptr_t.h:*{{expected function body}} \
+                       c99-modules-error@module.modulemap:*{{'_Builtin_stddef.nullptr_t' requires feature 'c23'}} c99-modules-note@stddef.h:*{{submodule of top-level module '_Builtin_stddef' implicitly imported here}}
 
 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}}
+nullptr_t n6; // c99-error{{unknown type}} c99-modules-error{{unknown type}}
+static void f6(void) { unreachable(); } // c99-error{{undeclared function}} c23-error{{undeclared identifier}} \
+                                           c99-modules-error{{undeclared function}} c23-modules-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}}
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}} \
+                                                 c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}} \
+                                                 c23-modules-error{{undeclared identifier}} c23-modules-error{{expected expression}} c23-modules-error{{undeclared identifier}}
 wint_t wi6; // c99-error{{unknown type}} c23-error{{unknown type}}
 
 #define __need_unreachable
@@ -118,11 +177,13 @@
 rsize_t r7;
 wchar_t wc7;
 void *v7 = NULL;
-nullptr_t n7 ; // c99-error{{unknown type}}
+nullptr_t n7 ; // c99-error{{unknown type}} c99-modules-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}}
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}} \
+                                                 c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}} \
+                                                 c23-modules-error{{undeclared identifier}} c23-modules-error{{expected expression}} c23-modules-error{{undeclared identifier}}
 wint_t wi7; // c99-error{{unknown type}} c23-error{{unknown type}}
 
 #define __need_max_align_t
@@ -133,11 +194,13 @@
 rsize_t r8;
 wchar_t wc8;
 void *v8 = NULL;
-nullptr_t n8; // c99-error{{unknown type}}
+nullptr_t n8; // c99-error{{unknown type}} c99-modules-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}}
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}} \
+                                                 c99-modules-error{{expected expression}} c99-modules-error{{undeclared identifier}} \
+                                                 c23-modules-error{{undeclared identifier}} c23-modules-error{{expected expression}} c23-modules-error{{undeclared identifier}}
 wint_t wi8; // c99-error{{unknown type}} c23-error{{unknown type}}
 
 #define __need_offsetof
@@ -146,7 +209,7 @@
 ptrdiff_t p9;
 size_t s9;
 rsize_t r9;
-nullptr_t n9; // c99-error{{unknown type}}
+nullptr_t n9; // c99-error{{unknown type}} c99-modules-error{{unknown type}}
 static void f9(void) { unreachable(); }
 wchar_t wc9;
 void *v9 = NULL;
@@ -162,7 +225,7 @@
 rsize_t r10;
 wchar_t wc10;
 void *v10 = NULL;
-nullptr_t n10; // c99-error{{unknown type}}
+nullptr_t n10; // c99-error{{unknown type}} c99-modules-error{{unknown type}}
 static void f10(void) { unreachable(); }
 max_align_t m10;
 size_t o10 = offsetof(struct astruct, member);