diff --git a/clang/lib/Basic/Module.cpp b/clang/lib/Basic/Module.cpp
index 740d2b7f57787..6a6c1d2414ce1 100644
--- a/clang/lib/Basic/Module.cpp
+++ b/clang/lib/Basic/Module.cpp
@@ -308,7 +308,8 @@ bool Module::directlyUses(const Module *Requested) {
   // Anyone is allowed to use our builtin stdarg.h and stddef.h and their
   // accompanying modules.
   if (Requested->getTopLevelModuleName() == "_Builtin_stdarg" ||
-      Requested->getTopLevelModuleName() == "_Builtin_stddef")
+      Requested->getTopLevelModuleName() == "_Builtin_stddef" ||
+      (!Requested->Parent && Requested->Name == "_Builtin_stddef_max_align_t"))
     return true;
 
   if (NoUndeclaredIncludes)
diff --git a/clang/lib/Headers/__stddef_max_align_t.h b/clang/lib/Headers/__stddef_max_align_t.h
index 512606a877288..e3b439285d0fd 100644
--- 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 ---------------===
+/*===---- __stddef_max_align_t.h - Definition of max_align_t for modules ---===
  *
  * 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
index b6e1dd1fd9707..33d0ec62fd905 100644
--- a/clang/lib/Headers/__stddef_null.h
+++ b/clang/lib/Headers/__stddef_null.h
@@ -7,15 +7,7 @@
  *===-----------------------------------------------------------------------===
  */
 
-#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
@@ -25,5 +17,3 @@
 #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
index 183d394d56c1b..8d23ed6dc8c69 100644
--- a/clang/lib/Headers/__stddef_nullptr_t.h
+++ b/clang/lib/Headers/__stddef_nullptr_t.h
@@ -7,9 +7,11 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef _NULLPTR_T
+#if !defined(_NULLPTR_T) || __has_feature(modules)
+/* Always define nullptr_t when modules are available. */
+#if !__has_feature(modules)
 #define _NULLPTR_T
-
+#endif
 #ifdef __cplusplus
 #if defined(_MSC_EXTENSIONS) && defined(_NATIVE_NULLPTR_SUPPORTED)
 namespace std {
@@ -17,8 +19,7 @@ typedef decltype(nullptr) nullptr_t;
 }
 using ::std::nullptr_t;
 #endif
-#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#else
 typedef typeof(nullptr) nullptr_t;
 #endif
-
 #endif
diff --git a/clang/lib/Headers/__stddef_offsetof.h b/clang/lib/Headers/__stddef_offsetof.h
index 3b347b3b92f62..84af8357c285b 100644
--- a/clang/lib/Headers/__stddef_offsetof.h
+++ b/clang/lib/Headers/__stddef_offsetof.h
@@ -7,6 +7,7 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef offsetof
+#if !defined(offsetof) || __has_feature(modules)
+/* Always define offsetof when modules are available. */
 #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
index 3ea6d7d2852e1..853cee5e09799 100644
--- a/clang/lib/Headers/__stddef_ptrdiff_t.h
+++ b/clang/lib/Headers/__stddef_ptrdiff_t.h
@@ -7,9 +7,10 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef _PTRDIFF_T
+#if !defined(_PTRDIFF_T) || __has_feature(modules)
+/* Always define ptrdiff_t when modules are available. */
+#if !__has_feature(modules)
 #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
index b6428d0c12b62..f9c31cf8f9746 100644
--- a/clang/lib/Headers/__stddef_rsize_t.h
+++ b/clang/lib/Headers/__stddef_rsize_t.h
@@ -7,9 +7,10 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef _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
diff --git a/clang/lib/Headers/__stddef_size_t.h b/clang/lib/Headers/__stddef_size_t.h
index e4a389510bcdb..b30e245e0eb5c 100644
--- a/clang/lib/Headers/__stddef_size_t.h
+++ b/clang/lib/Headers/__stddef_size_t.h
@@ -7,9 +7,10 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef _SIZE_T
+#if !defined(_SIZE_T) || __has_feature(modules)
+/* Always define size_t when modules are available. */
+#if !__has_feature(modules)
 #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
index 3e7fe01979662..5252932cd2769 100644
--- a/clang/lib/Headers/__stddef_unreachable.h
+++ b/clang/lib/Headers/__stddef_unreachable.h
@@ -7,6 +7,7 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef unreachable
+#if !defined(unreachable) || __has_feature(modules)
+/* Always define unreachable when modules are available. */
 #define unreachable() __builtin_unreachable()
 #endif
diff --git a/clang/lib/Headers/__stddef_wchar_t.h b/clang/lib/Headers/__stddef_wchar_t.h
index 16a6186512c0c..c3f0516a4b1ed 100644
--- a/clang/lib/Headers/__stddef_wchar_t.h
+++ b/clang/lib/Headers/__stddef_wchar_t.h
@@ -8,16 +8,14 @@
  */
 
 #if !defined(__cplusplus) || (defined(_MSC_VER) && !_NATIVE_WCHAR_T_DEFINED)
-
-#ifndef _WCHAR_T
+/* Always define wchar_t when modules are available. */
+#if !defined(_WCHAR_T) || __has_feature(modules)
+#if !__has_feature(modules)
 #define _WCHAR_T
-
-#ifdef _MSC_EXTENSIONS
+#if defined(_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
index 0aa2915079571..d0582ef971efd 100644
--- a/clang/lib/Headers/__stddef_wint_t.h
+++ b/clang/lib/Headers/__stddef_wint_t.h
@@ -7,9 +7,10 @@
  *===-----------------------------------------------------------------------===
  */
 
-#ifndef _WINT_T
+/* Always define wint_t when modules are available. */
+#if !defined(_WINT_T) || __has_feature(modules)
+#if !__has_feature(modules)
 #define _WINT_T
-
+#endif
 typedef __WINT_TYPE__ wint_t;
-
 #endif
diff --git a/clang/lib/Headers/module.modulemap b/clang/lib/Headers/module.modulemap
index a786689d39177..6894672ef0529 100644
--- a/clang/lib/Headers/module.modulemap
+++ b/clang/lib/Headers/module.modulemap
@@ -153,164 +153,9 @@ module _Builtin_intrinsics [system] [extern_c] {
   }
 }
 
-// Start -fbuiltin-headers-in-system-modules affected modules
-
-// The following modules all ignore their top level headers
-// when -fbuiltin-headers-in-system-modules is passed, and
-// most of those headers join system modules when present.
-
-// e.g. if -fbuiltin-headers-in-system-modules is passed, then
-// float.h will not be in the _Builtin_float module (that module
-// will be empty). If there is a system module that declares
-// `header "float.h"`, then the builtin float.h will join
-// that module. The system float.h (if present) will be treated
-// as a textual header in the sytem module.
-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 *
-}
-
-// When -fbuiltin-headers-in-system-modules is passed, only
-// the top level headers are removed, the implementation headers
-// will always be in their submodules. That means when stdarg.h
-// is included, it will still import this module and make the
-// appropriate submodules visible.
-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 {
-    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 *
-  }
-}
-
-/* wint_t is provided by <wchar.h> and not <stddef.h>. It's here
- * for compatibility, but must be explicitly requested. Therefore
- * __stddef_wint_t.h is not part of _Builtin_stddef. */
-module _Builtin_stddef_wint_t [system] {
-  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 _Builtin_stddef_max_align_t [system] [extern_c] {
+  header "__stddef_max_align_t.h"
 }
-// End -fbuiltin-headers-in-system-modules affected modules
 
 module opencl_c {
   requires opencl
diff --git a/clang/lib/Headers/stdarg.h b/clang/lib/Headers/stdarg.h
index 94b066566f084..085e2bb861ceb 100644
--- a/clang/lib/Headers/stdarg.h
+++ b/clang/lib/Headers/stdarg.h
@@ -7,31 +7,9 @@
  *===-----------------------------------------------------------------------===
  */
 
-/*
- * This header is designed to be included multiple times. If any of the __need_
- * macros are defined, then only that subset of interfaces are provided. This
- * can be useful for POSIX headers that need to not expose all of stdarg.h, but
- * need to use some of its interfaces. Otherwise this header provides all of
- * the expected interfaces.
- *
- * When clang modules are enabled, this header is a textual header. It ignores
- * its header guard so that multiple submodules can export its interfaces.
- * Take module SM with submodules A and B, whose headers both include stdarg.h
- * When SM.A builds, __STDARG_H will be defined. When SM.B builds, the
- * definition from SM.A will leak when building without local submodule
- * visibility. stdarg.h wouldn't include any of its implementation headers, and
- * SM.B wouldn't import any of the stdarg modules, and SM.B's `export *`
- * wouldn't export any stdarg interfaces as expected. However, since stdarg.h
- * ignores its header guard when building with modules, it all works as
- * expected.
- *
- * When clang modules are not enabled, the header guards can function in the
- * normal simple fashion.
- */
-#if !defined(__STDARG_H) || __has_feature(modules) ||                          \
-    defined(__need___va_list) || defined(__need_va_list) ||                    \
-    defined(__need_va_arg) || defined(__need___va_copy) ||                     \
-    defined(__need_va_copy)
+#if !defined(__STDARG_H) || defined(__need___va_list) ||                       \
+    defined(__need_va_list) || defined(__need_va_arg) ||                       \
+    defined(__need___va_copy) || defined(__need_va_copy)
 
 #if !defined(__need___va_list) && !defined(__need_va_list) &&                  \
     !defined(__need_va_arg) && !defined(__need___va_copy) &&                   \
diff --git a/clang/lib/Headers/stddef.h b/clang/lib/Headers/stddef.h
index 70e3ba2dd2e26..98ff869d4d155 100644
--- a/clang/lib/Headers/stddef.h
+++ b/clang/lib/Headers/stddef.h
@@ -7,41 +7,23 @@
  *===-----------------------------------------------------------------------===
  */
 
-/*
- * This header is designed to be included multiple times. If any of the __need_
- * macros are defined, then only that subset of interfaces are provided. This
- * can be useful for POSIX headers that need to not expose all of stddef.h, but
- * need to use some of its interfaces. Otherwise this header provides all of
- * the expected interfaces.
- *
- * When clang modules are enabled, this header is a textual header. It ignores
- * its header guard so that multiple submodules can export its interfaces.
- * Take module SM with submodules A and B, whose headers both include stddef.h
- * When SM.A builds, __STDDEF_H will be defined. When SM.B builds, the
- * definition from SM.A will leak when building without local submodule
- * visibility. stddef.h wouldn't include any of its implementation headers, and
- * SM.B wouldn't import any of the stddef modules, and SM.B's `export *`
- * wouldn't export any stddef interfaces as expected. However, since stddef.h
- * ignores its header guard when building with modules, it all works as
- * expected.
- *
- * When clang modules are not enabled, the header guards can function in the
- * normal simple fashion.
- */
-#if !defined(__STDDEF_H) || __has_feature(modules) ||                          \
-    (defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1) ||        \
-    defined(__need_ptrdiff_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)
+#if !defined(__STDDEF_H) || defined(__need_ptrdiff_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_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
diff --git a/clang/test/ClangScanDeps/modules-cas-fs-prefix-mapping-caching.c b/clang/test/ClangScanDeps/modules-cas-fs-prefix-mapping-caching.c
index 0fdf3779b826c..230b21b83fae7 100644
--- a/clang/test/ClangScanDeps/modules-cas-fs-prefix-mapping-caching.c
+++ b/clang/test/ClangScanDeps/modules-cas-fs-prefix-mapping-caching.c
@@ -21,24 +21,20 @@
 // RUN:  > %t/dir2.txt
 
 // Extract individual commands.
-// RUN: %deps-to-rsp %t/dir1.txt --module-name=_Builtin_stdarg > %t/dir1/stdarg.cc1.rsp
 // RUN: %deps-to-rsp %t/dir1.txt --module-name=B > %t/dir1/B.cc1.rsp
 // RUN: %deps-to-rsp %t/dir1.txt --module-name=A > %t/dir1/A.cc1.rsp
 // RUN: %deps-to-rsp %t/dir1.txt --tu-index 0 > %t/dir1/tu.cc1.rsp
 
-// RUN: %deps-to-rsp %t/dir2.txt --module-name=_Builtin_stdarg > %t/dir2/stdarg.cc1.rsp
 // RUN: %deps-to-rsp %t/dir2.txt --module-name=B > %t/dir2/B.cc1.rsp
 // RUN: %deps-to-rsp %t/dir2.txt --module-name=A > %t/dir2/A.cc1.rsp
 // RUN: %deps-to-rsp %t/dir2.txt --tu-index 0 > %t/dir2/tu.cc1.rsp
 
-// RUN: (cd %t/dir1; %clang @stdarg.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-MISS
 // RUN: (cd %t/dir1; %clang @B.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-MISS
 // RUN: (cd %t/dir1; %clang @A.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-MISS
 // RUN: (cd %t/dir1; %clang @tu.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-MISS
 
 // CACHE-MISS: compile job cache miss
 
-// RUN: (cd %t/dir2; %clang @stdarg.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-HIT
 // RUN: (cd %t/dir2; %clang @B.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-HIT
 // RUN: (cd %t/dir2; %clang @A.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-HIT
 // RUN: (cd %t/dir2; %clang @tu.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-HIT
diff --git a/clang/test/ClangScanDeps/modules-cas-fs-prefix-mapping.c b/clang/test/ClangScanDeps/modules-cas-fs-prefix-mapping.c
index 9d04469028760..50f99f4fa609a 100644
--- a/clang/test/ClangScanDeps/modules-cas-fs-prefix-mapping.c
+++ b/clang/test/ClangScanDeps/modules-cas-fs-prefix-mapping.c
@@ -16,7 +16,6 @@
 // RUN: FileCheck %s -input-file %t/full_result.txt -DPREFIX=%t -DSDK_PREFIX=%S/Inputs/SDK
 
 // Extract individual commands.
-// RUN: %deps-to-rsp %t/full_result.txt --module-name=_Builtin_stdarg > %t/stdarg.cc1.rsp
 // RUN: %deps-to-rsp %t/full_result.txt --module-name=B > %t/B.cc1.rsp
 // RUN: %deps-to-rsp %t/full_result.txt --module-name=A > %t/A.cc1.rsp
 // RUN: %deps-to-rsp %t/full_result.txt --tu-index 0 > %t/tu.cc1.rsp
@@ -42,7 +41,6 @@
 // FS: file llvmcas://{{.*}} /^tc/lib/clang/{{.*}}/include/stdarg.h
 
 // Check that it builds.
-// RUN: %clang @%t/stdarg.cc1.rsp
 // RUN: %clang @%t/B.cc1.rsp
 // RUN: %clang @%t/A.cc1.rsp
 // RUN: %clang @%t/tu.cc1.rsp
@@ -91,11 +89,7 @@
 // CHECK:          }
 // CHECK:          {
 // CHECK:            "casfs-root-id": "[[B_ROOT_ID:llvmcas://[[:xdigit:]]+]]"
-// CHECK:            "clang-module-deps": [
-// CHECK:              {
-// CHECK:                "module-name": "_Builtin_stdarg"
-// CHECK:              }
-// CHECK:            ],
+// CHECK:            "clang-module-deps": [],
 // CHECK:            "clang-modulemap-file": "[[PREFIX]]/module.modulemap"
 // CHECK:            "command-line": [
 // CHECK:              "-fcas-path"
diff --git a/clang/test/ClangScanDeps/modules-pch-cas-fs-prefix-mapping-caching.c b/clang/test/ClangScanDeps/modules-pch-cas-fs-prefix-mapping-caching.c
index a1bff204c22b5..3341c048b87ee 100644
--- a/clang/test/ClangScanDeps/modules-pch-cas-fs-prefix-mapping-caching.c
+++ b/clang/test/ClangScanDeps/modules-pch-cas-fs-prefix-mapping-caching.c
@@ -22,22 +22,18 @@
 // RUN:  > %t/pch_dir2.txt
 
 // == Build PCH
-// RUN: %deps-to-rsp %t/pch_dir1.txt --module-name=_Builtin_stdarg > %t/dir1/stdarg.cc1.rsp
 // RUN: %deps-to-rsp %t/pch_dir1.txt --module-name=B > %t/dir1/B.cc1.rsp
 // RUN: %deps-to-rsp %t/pch_dir1.txt --module-name=A > %t/dir1/A.cc1.rsp
 // RUN: %deps-to-rsp %t/pch_dir1.txt --tu-index 0 > %t/dir1/pch.cc1.rsp
-// RUN: (cd %t/dir1; %clang @stdarg.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-MISS
 // RUN: (cd %t/dir1; %clang @B.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-MISS
 // RUN: (cd %t/dir1; %clang @A.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-MISS
 // RUN: (cd %t/dir1; %clang @pch.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-MISS
 
 // CACHE-MISS: compile job cache miss
 
-// RUN: %deps-to-rsp %t/pch_dir2.txt --module-name=_Builtin_stdarg > %t/dir2/stdarg.cc1.rsp
 // RUN: %deps-to-rsp %t/pch_dir2.txt --module-name=B > %t/dir2/B.cc1.rsp
 // RUN: %deps-to-rsp %t/pch_dir2.txt --module-name=A > %t/dir2/A.cc1.rsp
 // RUN: %deps-to-rsp %t/pch_dir2.txt --tu-index 0 > %t/dir2/pch.cc1.rsp
-// RUN: (cd %t/dir2; %clang @stdarg.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-HIT
 // RUN: (cd %t/dir2; %clang @B.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-HIT
 // RUN: (cd %t/dir2; %clang @A.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-HIT
 // RUN: (cd %t/dir2; %clang @pch.cc1.rsp) 2>&1 | FileCheck %s -check-prefix=CACHE-HIT
diff --git a/clang/test/ClangScanDeps/modules-pch-cas-fs-prefix-mapping.c b/clang/test/ClangScanDeps/modules-pch-cas-fs-prefix-mapping.c
index dbd5869dde4fa..4b0a436e8e217 100644
--- a/clang/test/ClangScanDeps/modules-pch-cas-fs-prefix-mapping.c
+++ b/clang/test/ClangScanDeps/modules-pch-cas-fs-prefix-mapping.c
@@ -18,11 +18,9 @@
 // RUN: FileCheck %s -input-file %t/pch_result.txt -DPREFIX=%t -DSDK_PREFIX=%S/Inputs/SDK -check-prefix=PCH
 
 // == Build PCH
-// RUN: %deps-to-rsp %t/pch_result.txt --module-name=_Builtin_stdarg > %t/stdarg.cc1.rsp
 // RUN: %deps-to-rsp %t/pch_result.txt --module-name=B > %t/B.cc1.rsp
 // RUN: %deps-to-rsp %t/pch_result.txt --module-name=A > %t/A.cc1.rsp
 // RUN: %deps-to-rsp %t/pch_result.txt --tu-index 0 > %t/pch.cc1.rsp
-// RUN: %clang @%t/stdarg.cc1.rsp
 // RUN: %clang @%t/B.cc1.rsp
 // RUN: %clang @%t/A.cc1.rsp
 // Ensure we load pcms from action cache
@@ -121,11 +119,7 @@
 // PCH:          }
 // PCH:          {
 // PCH:            "casfs-root-id": "[[B_ROOT_ID:llvmcas://[[:xdigit:]]+]]"
-// PCH:            "clang-module-deps": [
-// PCH:               {
-// PCH:                 "module-name": "_Builtin_stdarg"
-// PCH:               }
-// PCH:             ],
+// PCH:            "clang-module-deps": [],
 // PCH:            "clang-modulemap-file": "[[PREFIX]]/module.modulemap"
 // PCH:            "command-line": [
 // PCH:              "-fcas-path"
diff --git a/clang/test/Headers/stdarg.c b/clang/test/Headers/stdarg.c
index 1c403d109bf7c..49df42caa3300 100644
--- a/clang/test/Headers/stdarg.c
+++ b/clang/test/Headers/stdarg.c
@@ -1,47 +1,29 @@
-// RUN: rm -fR %t
 // 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}} \
-                         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}}
+    __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}}
     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}} \
-                       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}}
+                       c99-error{{call to undeclared library function 'va_start'}} c99-note{{provide a declaration}} c99-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}} \
-                               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}}
+                               c99-error{{call to undeclared function 'va_arg'}} c99-error{{expected expression}} c99-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}} \
-                  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}}
+                  c99-error{{call to undeclared library function 'va_end'}} c99-note{{provide a declaration}} c99-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}} \
-                        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}}
+                        c99-error{{call to undeclared function '__va_copy'}} c99-error{{undeclared identifier}} c99-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}} \
-                      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}}
+                      c99-error{{call to undeclared library function 'va_copy'}} c99-note{{provide a declaration}} c99-error{{undeclared identifier}} c99-error{{undeclared identifier}}
 }
 
 //--- stdarg1.c
 // c99-no-diagnostics
-// c99-modules-no-diagnostics
 
 #include <stdarg.h>
 static void f(int p, ...) {
@@ -51,6 +33,5 @@ static void f(int p, ...) {
     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}} \
-                      c89-modules-error{{implicitly declaring library function}} c89-modules-note{{provide a declaration}}
+    va_copy(g, v); // c89-error{{implicitly declaring library function}} c89-note{{provide a declaration}}
 }
diff --git a/clang/test/Headers/stdargneeds.c b/clang/test/Headers/stdargneeds.c
index 8ef76e90deaef..7505ff55731f1 100644
--- a/clang/test/Headers/stdargneeds.c
+++ b/clang/test/Headers/stdargneeds.c
@@ -1,35 +1,23 @@
-// RUN: rm -fR %t
 // 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'}} 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}}
+    __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'}}
 }
 
 //--- stdargneeds1.c
@@ -37,37 +25,25 @@ static void f(int p, ...) {
 #include <stdarg.h>
 static void f(int p, ...) {
     __gnuc_va_list g;
-    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}}
+    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}}
 }
 
 //--- stdargneeds2.c
 #define __need_va_list
 #include <stdarg.h>
 static void f(int p, ...) {
-    __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}}
+    __gnuc_va_list g; // expected-error{{undeclared identifier}}
     va_list v;
-    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}}
+    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}}
 }
 
 //--- stdargneeds3.c
@@ -75,16 +51,13 @@ static void f(int p, ...) {
 #define __need_va_arg
 #include <stdarg.h>
 static void f(int p, ...) {
-    __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}}
+    __gnuc_va_list g; // expected-error{{undeclared identifier}}
     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}} \
-                        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}}
+    __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}}
 }
 
 //--- stdargneeds4.c
@@ -95,15 +68,11 @@ static void f(int p, ...) {
 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}} \
-                       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_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);
-    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}}
+    va_copy(g, v); // expected-error{{implicitly declaring library function}} expected-note{{provide a declaration}}
 }
 
 //--- stdargneeds5.c
@@ -114,12 +83,9 @@ static void f(int p, ...) {
 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}} \
-                       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_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_copy(g, v);
 }
diff --git a/clang/test/Headers/stddef.c b/clang/test/Headers/stddef.c
index 8a8968858e03d..30ccfa7a31e07 100644
--- a/clang/test/Headers/stddef.c
+++ b/clang/test/Headers/stddef.c
@@ -1,57 +1,35 @@
-// RUN: rm -fR %t
 // 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=c2x %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=c2x %s
 
 struct astruct { char member; };
 
-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}}
+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}} \
-                                                 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}}
+                                                 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_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}}
+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}}
 wchar_t wc1;
 void *v1 = NULL;
-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}}
+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}} \
-               c99-modules-error{{unknown type}} c11-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+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
@@ -61,14 +39,8 @@ size_t s2;
 rsize_t r2;
 wchar_t wc2;
 void *v2 = NULL;
-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}}
+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}} \
-               c99-modules-error{{unknown type}} c11-modules-error{{unknown type}} c23-modules-error{{unknown type}}
-
-// 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.
+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
index 194bf0a9c8093..28f57e0719c8f 100644
--- a/clang/test/Headers/stddefneeds.c
+++ b/clang/test/Headers/stddefneeds.c
@@ -1,72 +1,36 @@
-// RUN: rm -fR %t
 // RUN: %clang_cc1 -fsyntax-only -verify=c99 -std=c99 %s
 // RUN: %clang_cc1 -fsyntax-only -verify=c23 -std=c2x %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=c2x %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}} \
-                 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}}
+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'}} \
-                                                 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}}
+                                                 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}} \
-              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 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}} \
-                                                 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{{unknown type}} c23-modules-error{{unknown type}}
-
-// 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.
+                                                 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>
@@ -76,19 +40,13 @@ size_t s2;
 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}} \
-                    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}}
+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}} \
-                                                 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}} \
-               c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+                                                 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>
@@ -97,19 +55,13 @@ 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}} \
-                    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}}
+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}} \
-                                                 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}} \
-               c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+                                                 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>
@@ -118,19 +70,13 @@ ptrdiff_t p4;
 size_t s4;
 rsize_t r4;
 wchar_t wc4;
-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}}
+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}} \
-                                                 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}} \
-               c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+                                                 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>
@@ -140,38 +86,29 @@ size_t s5;
 rsize_t r5;
 wchar_t wc5;
 void *v5 = NULL;
-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}}
+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}} \
-                                                 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}} \
-               c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+                                                 c23-error{{undeclared identifier}} c23-error{{expected expression}} c23-error{{undeclared identifier}}
+wint_t wi5; // c99-error{{unknown type}} c23-error{{unknown type}}
 
-// nullptr_t doesn't get declared before C23 because its definition
+// __need_nullptr_t generates an error in <C23 because its definition
 // depends on nullptr.
 #define __need_nullptr_t
-#include <stddef.h>
+#include <stddef.h> // c99-error@__stddef_nullptr_t.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}} 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}}
+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}} \
-                                                 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}} \
-               c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+                                                 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>
@@ -181,15 +118,12 @@ size_t s7;
 rsize_t r7;
 wchar_t wc7;
 void *v7 = NULL;
-nullptr_t n7 ; // c99-error{{unknown type}} c99-modules-error{{unknown type}}
+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}} \
-                                                 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}} \
-               c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+                                                 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>
@@ -199,15 +133,12 @@ size_t s8;
 rsize_t r8;
 wchar_t wc8;
 void *v8 = NULL;
-nullptr_t n8; // c99-error{{unknown type}} c99-modules-error{{unknown type}}
+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}} \
-                                                 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}} \
-               c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+                                                 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>
@@ -215,14 +146,13 @@ wint_t wi8; // c99-error{{unknown type}} c23-error{{unknown type}} \
 ptrdiff_t p9;
 size_t s9;
 rsize_t r9;
-nullptr_t n9; // c99-error{{unknown type}} c99-modules-error{{unknown type}}
+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}} \
-               c99-modules-error{{unknown type}} c23-modules-error{{unknown type}}
+wint_t wi9; // c99-error{{unknown type}} c23-error{{unknown type}}
 
 #define __need_wint_t
 #include <stddef.h>
@@ -232,7 +162,7 @@ size_t s10;
 rsize_t r10;
 wchar_t wc10;
 void *v10 = NULL;
-nullptr_t n10; // c99-error{{unknown type}} c99-modules-error{{unknown type}}
+nullptr_t n10; // c99-error{{unknown type}}
 static void f10(void) { unreachable(); }
 max_align_t m10;
 size_t o10 = offsetof(struct astruct, member);
diff --git a/clang/test/Modules/Inputs/builtin-headers/builtin-modules.modulemap b/clang/test/Modules/Inputs/builtin-headers/builtin-modules.modulemap
deleted file mode 100644
index 0fbb454dbb022..0000000000000
--- a/clang/test/Modules/Inputs/builtin-headers/builtin-modules.modulemap
+++ /dev/null
@@ -1,29 +0,0 @@
-module c_complex [system] {
-  header "complex.h"
-  export *
-}
-
-module c_float [system] {
-  header "float.h"
-  export *
-}
-
-module c_inttypes [system] {
-  header "inttypes.h"
-  export *
-}
-
-module c_limits [system] {
-  header "limits.h"
-  export *
-}
-
-module c_math [system] {
-  header "math.h"
-  export *
-}
-
-module c_stdint [system] {
-  header "stdint.h"
-  export *
-}
diff --git a/clang/test/Modules/Inputs/builtin-headers/c++/module.modulemap b/clang/test/Modules/Inputs/builtin-headers/c++/module.modulemap
deleted file mode 100644
index c969f067a234f..0000000000000
--- a/clang/test/Modules/Inputs/builtin-headers/c++/module.modulemap
+++ /dev/null
@@ -1,4 +0,0 @@
-module cpp_stdint [system] {
-  header "stdint.h"
-  export *
-}
diff --git a/clang/test/Modules/Inputs/builtin-headers/c++/stdint.h b/clang/test/Modules/Inputs/builtin-headers/c++/stdint.h
deleted file mode 100644
index 22cebacfdb64e..0000000000000
--- a/clang/test/Modules/Inputs/builtin-headers/c++/stdint.h
+++ /dev/null
@@ -1,6 +0,0 @@
-#ifndef CPP_STDINT_H
-#define CPP_STDINT_H
-
-#include_next <stdint.h>
-
-#endif
diff --git a/clang/test/Modules/Inputs/builtin-headers/complex.h b/clang/test/Modules/Inputs/builtin-headers/complex.h
deleted file mode 100644
index b6ef3c5a35b45..0000000000000
--- a/clang/test/Modules/Inputs/builtin-headers/complex.h
+++ /dev/null
@@ -1 +0,0 @@
-// Required by tgmath.h
diff --git a/clang/test/Modules/Inputs/builtin-headers/float.h b/clang/test/Modules/Inputs/builtin-headers/float.h
deleted file mode 100644
index e69de29bb2d1d..0000000000000
diff --git a/clang/test/Modules/Inputs/builtin-headers/inttypes.h b/clang/test/Modules/Inputs/builtin-headers/inttypes.h
deleted file mode 100644
index ddb6bb83b5723..0000000000000
--- a/clang/test/Modules/Inputs/builtin-headers/inttypes.h
+++ /dev/null
@@ -1,12 +0,0 @@
-#ifndef INTTYPES_H
-#define INTTYPES_H
-
-// This creates an include cycle when inttypes.h and stdint.h
-// are both part of the cstd module. This include will resolve
-// to the C++ stdint.h, which will #include_next eventually to
-// the stdint.h in this directory, and thus create the cycle
-// cstd (inttypes.h) -> cpp_stdint (stdint.h) -> cstd (stdint.h).
-// This cycle is worked around by cstd using [no_undeclared_includes].
-#include <stdint.h>
-
-#endif
diff --git a/clang/test/Modules/Inputs/builtin-headers/limits.h b/clang/test/Modules/Inputs/builtin-headers/limits.h
deleted file mode 100644
index 8b137891791fe..0000000000000
--- a/clang/test/Modules/Inputs/builtin-headers/limits.h
+++ /dev/null
@@ -1 +0,0 @@
-
diff --git a/clang/test/Modules/Inputs/builtin-headers/math.h b/clang/test/Modules/Inputs/builtin-headers/math.h
deleted file mode 100644
index b6ef3c5a35b45..0000000000000
--- a/clang/test/Modules/Inputs/builtin-headers/math.h
+++ /dev/null
@@ -1 +0,0 @@
-// Required by tgmath.h
diff --git a/clang/test/Modules/Inputs/builtin-headers/stdint.h b/clang/test/Modules/Inputs/builtin-headers/stdint.h
deleted file mode 100644
index 8f233fd7f45d6..0000000000000
--- a/clang/test/Modules/Inputs/builtin-headers/stdint.h
+++ /dev/null
@@ -1,34 +0,0 @@
-#ifndef STDINT_H
-#define STDINT_H
-
-// types needed by stdatomic.h
-
-typedef char int_least8_t;
-typedef short int_least16_t;
-typedef int int_least32_t;
-typedef long long int int_least64_t;
-typedef unsigned char uint_least8_t;
-typedef unsigned short uint_least16_t;
-typedef unsigned int uint_least32_t;
-typedef unsigned long long uint_least64_t;
-
-typedef char int_fast8_t;
-typedef short int_fast16_t;
-typedef int int_fast32_t;
-typedef long long int int_fast64_t;
-typedef unsigned char uint_fast8_t;
-typedef unsigned short uint_fast16_t;
-typedef unsigned int uint_fast32_t;
-typedef unsigned long long uint_fast64_t;
-
-typedef int intptr_t;
-typedef unsigned int uintptr_t;
-typedef int intmax_t;
-typedef unsigned int uintmax_t;
-
-// additional types for unwind.h
-
-typedef unsigned int uint32_t;
-typedef unsigned long long uint64_t;
-
-#endif
diff --git a/clang/test/Modules/Inputs/builtin-headers/system-modules.modulemap b/clang/test/Modules/Inputs/builtin-headers/system-modules.modulemap
deleted file mode 100644
index 0161ff80fe618..0000000000000
--- a/clang/test/Modules/Inputs/builtin-headers/system-modules.modulemap
+++ /dev/null
@@ -1,71 +0,0 @@
-module cstd [system] [no_undeclared_includes] {
-  module complex {
-    header "complex.h"
-    export *
-  }
-
-  module float {
-    header "float.h"
-    export *
-  }
-
-  module inttypes {
-    header "inttypes.h"
-    export *
-  }
-
-  module iso646 {
-    header "iso646.h"
-    export *
-  }
-
-  module limits {
-    header "limits.h"
-    export *
-  }
-
-  module math {
-    header "math.h"
-    export *
-  }
-
-  module stdalign {
-    header "stdalign.h"
-    export *
-  }
-
-  module stdarg {
-    header "stdarg.h"
-    export *
-  }
-
-  module stdatomic {
-    header "stdatomic.h"
-    export *
-  }
-
-  module stdbool {
-    header "stdbool.h"
-    export *
-  }
-
-  module stddef {
-    header "stddef.h"
-    export *
-  }
-
-  module stdint {
-    header "stdint.h"
-    export *
-  }
-
-  module tgmath {
-    header "tgmath.h"
-    export *
-  }
-
-  module unwind {
-    header "unwind.h"
-    export *
-  }
-}
diff --git a/clang/test/Modules/builtin-headers.mm b/clang/test/Modules/builtin-headers.mm
deleted file mode 100644
index 4c9ddec4e99c2..0000000000000
--- a/clang/test/Modules/builtin-headers.mm
+++ /dev/null
@@ -1,41 +0,0 @@
-// RUN: rm -rf %t
-// RUN: %clang_cc1 -cxx-isystem %S/Inputs/builtin-headers/c++ -internal-isystem %S/Inputs/builtin-headers -fsyntax-only -fmodules -fmodules-cache-path=%t -fmodule-map-file=%S/Inputs/builtin-headers/c++/module.modulemap -fmodule-map-file=%resource_dir/module.modulemap -fmodule-map-file=%S/Inputs/builtin-headers/system-modules.modulemap -fbuiltin-headers-in-system-modules -DSYSTEM_MODULES %s -verify
-// RUN: rm -rf %t
-// RUN: %clang_cc1 -cxx-isystem %S/Inputs/builtin-headers/c++ -internal-isystem %S/Inputs/builtin-headers -fsyntax-only -fmodules -fmodules-cache-path=%t -fmodule-map-file=%S/Inputs/builtin-headers/c++/module.modulemap -fmodule-map-file=%resource_dir/module.modulemap -fmodule-map-file=%S/Inputs/builtin-headers/builtin-modules.modulemap %s -verify
-
-// expected-no-diagnostics
-
-@import cpp_stdint;
-
-// The builtin modules are always available, though they're mostly
-// empty if -fbuiltin-headers-in-system-modules is used.
-@import _Builtin_float;
-@import _Builtin_inttypes;
-@import _Builtin_iso646;
-@import _Builtin_limits;
-@import _Builtin_stdalign;
-@import _Builtin_stdarg;
-@import _Builtin_stdatomic;
-@import _Builtin_stdbool;
-@import _Builtin_stddef;
-@import _Builtin_stdint;
-@import _Builtin_stdnoreturn;
-@import _Builtin_tgmath;
-@import _Builtin_unwind;
-
-#ifdef SYSTEM_MODULES
-// system-modules.modulemap uses the "mega module" style with
-// -fbuiltin-headers-in-system-modules, and its modules cover
-// the clang builtin headers.
-@import cstd;
-#else
-// builtin-modules.modulemap uses top level modules for each
-// of its headers, which allows interleaving with the builtin
-// modules and libc++ modules.
-@import c_complex;
-@import c_float;
-@import c_inttypes;
-@import c_limits;
-@import c_math;
-@import c_stdint;
-#endif
diff --git a/clang/test/Modules/stddef.c b/clang/test/Modules/stddef.c
index 5bc0d1e44c856..6704abffbf0b7 100644
--- a/clang/test/Modules/stddef.c
+++ b/clang/test/Modules/stddef.c
@@ -1,29 +1,13 @@
 // RUN: rm -rf %t
 // RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t -I%S/Inputs/StdDef %s -verify -fno-modules-error-recovery
-// RUN: rm -rf %t
-// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I%S/Inputs/StdDef %s -verify -fno-modules-error-recovery
 
 #include "ptrdiff_t.h"
 
 ptrdiff_t pdt;
 
-// size_t is declared in both size_t.h and __stddef_size_t.h, both of which are
-// modular headers. Regardless of whether stddef.h joins the StdDef test module
-// or is in its _Builtin_stddef module, __stddef_size_t.h will be in
-// _Builtin_stddef.size_t. It's not defined which module will win as the expected
-// provider of size_t. For the purposes of this test it doesn't matter which header
-// gets reported, just as long as it isn't other.h or include_again.h.
-size_t st; // expected-error-re {{missing '#include "{{size_t|__stddef_size_t}}.h"'; 'size_t' must be declared before it is used}}
-// expected-note@size_t.h:* 0+ {{here}}
-// expected-note@__stddef_size_t.h:* 0+ {{here}}
+size_t st; // expected-error {{missing '#include "include_again.h"'; 'size_t' must be declared before it is used}}
+// expected-note@__stddef_size_t.h:* {{here}}
 
 #include "include_again.h"
-// Includes <stddef.h> which includes <__stddef_size_t.h> which imports the
-// _Builtin_stddef.size_t module.
 
 size_t st2;
-
-#include "size_t.h"
-// Redeclares size_t, but the type merger should figure it out.
-
-size_t st3;
diff --git a/clang/test/Modules/stddef.m b/clang/test/Modules/stddef.m
index 855464afe61af..1ff6ff73796ab 100644
--- a/clang/test/Modules/stddef.m
+++ b/clang/test/Modules/stddef.m
@@ -4,6 +4,4 @@
 
 // RUN: rm -rf %t
 // RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fbuiltin-headers-in-system-modules -fmodules-cache-path=%t -I %S/Inputs/StdDef %s -verify
-// RUN: rm -rf %t
-// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/StdDef %s -verify
 // expected-no-diagnostics