diff --git a/libcxx/include/ext/__hash b/libcxx/include/ext/__hash
--- a/libcxx/include/ext/__hash
+++ b/libcxx/include/ext/__hash
@@ -14,6 +14,7 @@
 
 #include <__config>
 #include <cstring>
+#include <stddef.h>
 #include <string>
 
 namespace __gnu_cxx {