diff --git a/clang/lib/Headers/vadefs.h b/clang/lib/Headers/vadefs.h
index cb997fbf66c4..7fe9a74e3f69 100644
--- a/clang/lib/Headers/vadefs.h
+++ b/clang/lib/Headers/vadefs.h
@@ -45,5 +45,21 @@
 #define _crt_va_arg(ap, type)    __builtin_va_arg(ap, type)
 #endif
 
+/* VS 2015 switched to double underscore names, which is an improvement, but now
+ * we have to intercept those names too.
+ */
+#ifdef __crt_va_start
+#undef __crt_va_start
+#define __crt_va_start(ap, param) __builtin_va_start(ap, param)
+#endif
+#ifdef __crt_va_end
+#undef __crt_va_end
+#define __crt_va_end(ap)          __builtin_va_end(ap)
+#endif
+#ifdef __crt_va_arg
+#undef __crt_va_arg
+#define __crt_va_arg(ap, type)    __builtin_va_arg(ap, type)
+#endif
+
 #endif
 #endif