Guard __gnuc_va_list typedef

[sfertile]

I think the fix/test looks good, but someone with more experience than me should review it before accepting it

[yaron.keren]

LGTM, matches the code in libstdc++ stdarg,h.
You can remove the 'hack' comment in line 46, __GNUC_VA_LIST is just a standard include guard for the typedef.

lib/Headers/stdarg.h

 
 #if __STDC_VERSION__ >= 199901L || __cplusplus >= 201103L || !defined(__STRICT_ANSI__)
 #define va_copy(dest, src)  __builtin_va_copy(dest, src)
 #endif
 
 /* Hack required to make standard headers work, at least on Ubuntu */
 #ifndef __GNUC_VA_LIST
 #define __GNUC_VA_LIST 1
-#endif
 typedef __builtin_va_list __gnuc_va_list;
+#endif
 
 #endif /* __STDARG_H */

test/Headers/stdarg-gnuc_va_list.c

+// RUN: %clang_cc1 -fsyntax-only -verify -Wsystem-headers -std=c99 %s
+// expected-no-diagnostics
+
+// Check that no warnings are emitted from stdarg.h if __gnuc_va_list has
+// previously been defined in another header file.
+typedef __builtin_va_list __va_list;
+typedef __va_list __gnuc_va_list;
+#define __GNUC_VA_LIST
+
+#include <stdarg.h>