LWG Issue 2148: Hashing Enums

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189831 91177308-0d34-0410-b5e6-96231b3b80d8

include/functional

@@ -2399,6 +2399,22 @@ struct _LIBCPP_TYPE_VIS_ONLY hash<long double>
     }
 };
 
+#if _LIBCPP_STD_VER > 11
+template <class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY hash
+    : public unary_function<_Tp, size_t>
+{
+    static_assert(is_enum<_Tp>::value, "This hash only works for enumeration types");
+
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(_Tp __v) const _NOEXCEPT
+    {
+        typedef typename underlying_type<_Tp>::type type;
+        return hash<type>{}(static_cast<type>(__v));
+    }
+};
+#endif
+
 // struct hash<T*> in <memory>
 
 _LIBCPP_END_NAMESPACE_STD