Forward declare std::align_val_t

include/boost/container/detail/std_fwd.hpp

@@ -23,6 +23,8 @@
 //                        Standard predeclarations
 //////////////////////////////////////////////////////////////////////////////
 
+#include <cstddef>
+
 #include <boost/move/detail/std_ns_begin.hpp>
 BOOST_MOVE_STD_NS_BEG
 
@@ -59,4 +61,15 @@ struct pointer_traits;
 BOOST_MOVE_STD_NS_END
 #include <boost/move/detail/std_ns_end.hpp>
 
+#if defined(__cpp_aligned_new)
+
+//align_val_t is not usually in an inline namespace
+namespace std {
+
+enum class align_val_t : std::size_t;
+
+}  //namespace std
+
+#endif
+
 #endif //#ifndef BOOST_CONTAINER_DETAIL_STD_FWD_HPP