diff --git a/clang/lib/AST/FormatString.cpp b/clang/lib/AST/FormatString.cpp
--- a/clang/lib/AST/FormatString.cpp
+++ b/clang/lib/AST/FormatString.cpp
@@ -748,6 +748,15 @@
     case LengthModifier::AsIntMax:
     case LengthModifier::AsSizeT:
     case LengthModifier::AsPtrDiff:
+      if (LM.getKind() == LengthModifier::AsSizeT &&
+          Target.getTriple().isOSMSVCRT() &&
+          !LO.isCompatibleWithMSVC(LangOptions::MSVC2015)) {
+        // The standard libraries before MSVC2015 didn't support the 'z' length
+        // modifier for size_t. So if the MS compatibility version is specified
+        // and less than that, reject.
+        return false;
+      }
+
       switch (CS.getKind()) {
         case ConversionSpecifier::dArg:
         case ConversionSpecifier::DArg:
diff --git a/clang/test/Sema/format-strings-ms.c b/clang/test/Sema/format-strings-ms.c
--- a/clang/test/Sema/format-strings-ms.c
+++ b/clang/test/Sema/format-strings-ms.c
@@ -1,4 +1,6 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -fms-compatibility -triple=i386-pc-win32 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -fms-compatibility -triple=i386-pc-win32 -fms-compatibility-version=18 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -fms-compatibility -triple=i386-pc-win32 -fms-compatibility-version=19 -DSIZE_T_OK %s
 // RUN: %clang_cc1 -fsyntax-only -verify -fms-compatibility -triple=i386-pc-win32 -Wformat-non-iso -DNON_ISO_WARNING %s
 
 int printf(const char *format, ...) __attribute__((format(printf, 1, 2)));
@@ -85,4 +87,11 @@
   scanf("%Z", p); // expected-warning{{invalid conversion specifier 'Z'}}
 }
 
+void size_t_test(size_t s) {
+  printf("%zu", s);
+#ifndef SIZE_T_OK
+  // expected-warning@-2 {{length modifier 'z' results in undefined behavior or no effect with 'u' conversion specifier}}
+#endif
+}
+
 #endif