Index: clang/lib/Headers/stdint.h
===================================================================
--- clang/lib/Headers/stdint.h
+++ clang/lib/Headers/stdint.h
@@ -664,7 +664,7 @@
 #define UINTPTR_MAX __UINTN_MAX(__INTPTR_WIDTH__)
 #define PTRDIFF_MIN  __INTN_MIN(__PTRDIFF_WIDTH__)
 #define PTRDIFF_MAX  __INTN_MAX(__PTRDIFF_WIDTH__)
-#define    SIZE_MAX __UINTN_MAX(__SIZE_WIDTH__)
+#define    SIZE_MAX ((__SIZE_TYPE__)__UINTN_MAX(__SIZE_WIDTH__))
 
 /* ISO9899:2011 7.20 (C11 Annex K): Define RSIZE_MAX if __STDC_WANT_LIB_EXT1__
  * is enabled. */
Index: clang/test/Headers/stdint-type-of-SIZE_MAX.cpp
===================================================================
--- /dev/null
+++ clang/test/Headers/stdint-type-of-SIZE_MAX.cpp
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only %s
+#include <stdint.h>
+#include <stddef.h>
+
+static_assert(__is_same(__typeof__(SIZE_MAX), size_t),
+              "SIZE_MAX should be size_t");