Index: cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticSemaKinds.td
@@ -7570,7 +7570,7 @@
 def err_os_log_format_not_string_constant : Error<
   "os_log() format argument is not a string constant">;
 def err_os_log_argument_too_big : Error<
-  "os_log() argument %d is too big (%d bytes, max %d)">;
+  "os_log() argument %0 is too big (%1 bytes, max %2)">;
 def warn_os_log_format_narg : Error<
  "os_log() '%%n' format specifier is not allowed">, DefaultError;
 
Index: cfe/trunk/test/SemaObjC/format-strings-oslog.m
===================================================================
--- cfe/trunk/test/SemaObjC/format-strings-oslog.m
+++ cfe/trunk/test/SemaObjC/format-strings-oslog.m
@@ -36,6 +36,9 @@
   wchar_t wcs[] = {'a', 0};
   __builtin_os_log_format(buf, "%S", wcs);
   printf("%S", wcs);
+
+  struct { char data[0x100]; } toobig;
+  __builtin_os_log_format(buf, "%s", toobig); // expected-error {{os_log() argument 2 is too big (256 bytes, max 255)}}
 }
 
 // Test os_log_format primitive with ObjC string literal format argument.