Index: libcxx/include/__support/ibm/xlocale.h
===================================================================
--- libcxx/include/__support/ibm/xlocale.h
+++ libcxx/include/__support/ibm/xlocale.h
@@ -107,7 +107,11 @@
   va_list ap_copy;
   // va_copy may not be provided by the C library in C++ 03 mode.
 #if defined(_LIBCPP_CXX03_LANG) && __has_builtin(__builtin_va_copy)
+#if defined(__MVS__) && !defined(_VARARG_EXT_)
+  __builtin_zos_va_copy(ap_copy, ap);
+#else
   __builtin_va_copy(ap_copy, ap);
+#endif
 #else
   va_copy(ap_copy, ap);
 #endif