Index: lib/Headers/inttypes.h
===================================================================
--- lib/Headers/inttypes.h
+++ lib/Headers/inttypes.h
@@ -27,8 +27,35 @@
 #error MSVC does not have inttypes.h prior to Visual Studio 2013
 #endif
 
+// C99 7.8.1 Macros for format specifiers
+//
+//  Footnote 191: C++ implementations should define these macros only when
+//  __STDC_FORMAT_MACROS is defined before <inttypes.h> is included.
+//
+// C++11 [c.files]p3:
+//
+//  The macros defined by <cinttypes> are provided unconditionally.
+//  In particular, the symbol __STDC_FORMAT_MACROS (mentioned
+//  in footnote 182 of the C standard) plays no role in C++.
+//
+// C11 removed the problematic footnotes.
+//
+// Work around this inconsistency by always defining those macros in C++ mode,
+// so that a C library implementation which follows the C99 standard can be
+// used in C++.
+
+# if defined(__cplusplus) && !defined(__STDC_FORMAT_MACROS)
+#  define __STDC_FORMAT_MACROS
+#  define __STDC_FORMAT_MACROS_DEFINED_BY_CLANG
+# endif
+
 #include_next <inttypes.h>
 
+# ifdef __STDC_FORMAT_MACROS_DEFINED_BY_CLANG
+#  undef __STDC_FORMAT_MACROS
+#  undef __STDC_FORMAT_MACROS_DEFINED_BY_CLANG
+# endif
+
 #if defined(_MSC_VER) && _MSC_VER < 1900
 /* MSVC headers define int32_t as int, but PRIx32 as "lx" instead of "x".
  * This triggers format warnings, so fix it up here. */