diff --git a/stdarg.h b/stdarg.h
index e26da00..d756a59 100644
--- a/stdarg.h
+++ b/stdarg.h
@@ -11,7 +11,9 @@ typedef __builtin_va_list va_list;
 #define va_start(v, l) __builtin_va_start(v, l)
 #define va_end(v) __builtin_va_end(v)
 #define va_arg(v, l) __builtin_va_arg(v, l)
-#define va_copy(d, s) __builtin_va_copy(d, s)
+#if (defined(__cplusplus) && (__cplusplus >= 201103L)) || (defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L))
+#  define va_copy(d, s) __builtin_va_copy(d, s)
+#endif
 
 #endif
 
diff --git a/stddef.h b/stddef.h
index e087eaa..b40d82d 100644
--- a/stddef.h
+++ b/stddef.h
@@ -8,12 +8,16 @@ typedef __PTRDIFF_TYPE__ ptrdiff_t;
 
 #ifndef __cplusplus
 typedef __WCHAR_TYPE__ wchar_t;
+
+/* XXX fix to use proper __STDC_VERSION__ number later on */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 202000L)
+typedef typeof(nullptr) nullptr_t;
+#endif
+
 #endif
 
 #ifdef __cplusplus
 typedef decltype(nullptr) nullptr_t;
-
-enum class byte : unsigned char {};
 #endif
 
 #ifndef __STDDEF_H_MACROS
@@ -31,6 +35,11 @@ enum class byte : unsigned char {};
 
 #define offsetof(s, m) __builtin_offsetof(s, m)
 
+/* XXX fix to use proper __STDC_VERSION__ number later on */
+#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 202000L)
+#  define unreachable() __builtin_unreachable()
+#endif
+
 #endif
 
 #endif