Index: cfe/trunk/lib/Headers/stdarg.h
===================================================================
--- cfe/trunk/lib/Headers/stdarg.h
+++ cfe/trunk/lib/Headers/stdarg.h
@@ -10,6 +10,11 @@
 #ifndef __STDARG_H
 #define __STDARG_H
 
+#if defined(_MSC_VER)
+/* Include otherwise unneeded header for MSVC compatibility. */
+#include <vcruntime.h>
+#endif
+
 #ifndef _VA_LIST
 typedef __builtin_va_list va_list;
 #define _VA_LIST
Index: cfe/trunk/lib/Headers/stddef.h
===================================================================
--- cfe/trunk/lib/Headers/stddef.h
+++ cfe/trunk/lib/Headers/stddef.h
@@ -18,6 +18,12 @@
 #if !__has_feature(modules)
 #define __STDDEF_H
 #endif
+
+#if defined(_MSC_VER)
+/* Include otherwise unneeded header for MSVC compatibility. */
+#include <corecrt.h>
+#endif
+
 #define __need_ptrdiff_t
 #define __need_size_t
 #define __need_wchar_t
Index: cfe/trunk/test/Headers/Inputs/ms-crt/corecrt.h
===================================================================
--- cfe/trunk/test/Headers/Inputs/ms-crt/corecrt.h
+++ cfe/trunk/test/Headers/Inputs/ms-crt/corecrt.h
@@ -0,0 +1 @@
+#pragma once
Index: cfe/trunk/test/Headers/Inputs/ms-crt/vcruntime.h
===================================================================
--- cfe/trunk/test/Headers/Inputs/ms-crt/vcruntime.h
+++ cfe/trunk/test/Headers/Inputs/ms-crt/vcruntime.h
@@ -0,0 +1 @@
+#pragma once
Index: cfe/trunk/test/Headers/c11.c
===================================================================
--- cfe/trunk/test/Headers/c11.c
+++ cfe/trunk/test/Headers/c11.c
@@ -2,7 +2,8 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c11 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c11 -fmodules -fmodules-cache-path=%t %s -D__STDC_WANT_LIB_EXT1__=1
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c11 -ffreestanding %s
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c11 -triple i686-pc-win32 -fms-compatibility-version=17.00 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c11 -triple i686-pc-win32 \
+// RUN:  -fms-compatibility-version=17.00 -isystem %S/Inputs/ms-crt %s
 
 noreturn int f(); // expected-error 1+{{}}
 
Index: cfe/trunk/test/Headers/ms-additional-includes.cpp
===================================================================
--- cfe/trunk/test/Headers/ms-additional-includes.cpp
+++ cfe/trunk/test/Headers/ms-additional-includes.cpp
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only --show-includes -triple i686-pc-win32 \
+// RUN:  -isystem %S/Inputs/ms-crt -fms-compatibility-version=17.00 %s \
+// RUN:  | FileCheck %s
+
+#include <stddef.h>
+// CHECK: including file:{{.*}}stddef.h
+// CHECK: including file:{{.*}}corecrt.h
+#include <stdarg.h>
+// CHECK: including file:{{.*}}stdarg.h
+// CHECK: including file:{{.*}}vcruntime.h
Index: cfe/trunk/test/Headers/ms-null-ms-header-vs-stddef.cpp
===================================================================
--- cfe/trunk/test/Headers/ms-null-ms-header-vs-stddef.cpp
+++ cfe/trunk/test/Headers/ms-null-ms-header-vs-stddef.cpp
@@ -1,4 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -triple i686-pc-win32 -fms-compatibility -fms-compatibility-version=17.00 %s
+// RUN: %clang_cc1 -fsyntax-only -triple i686-pc-win32 -fms-compatibility \
+// RUN:  -isystem %S/Inputs/ms-crt -fms-compatibility-version=17.00 %s
 // RUN: %clang_cc1 -fsyntax-only -triple i386-mingw32 %s
 
 // Something in MSVC's headers (pulled in e.g. by <crtdefs.h>) defines __null