updated is_avalanching trait protocol

include/boost/unordered/hash_traits.hpp

@@ -1,6 +1,6 @@
 /* Hash function characterization.
  *
- * Copyright 2022 Joaquin M Lopez Munoz.
+ * Copyright 2022-2024 Joaquin M Lopez Munoz.
  * Distributed under the Boost Software License, Version 1.0.
  * (See accompanying file LICENSE_1_0.txt or copy at
  * http://www.boost.org/LICENSE_1_0.txt)
@@ -19,12 +19,28 @@ namespace unordered{
 namespace detail{
 
 template<typename Hash,typename=void>
-struct hash_is_avalanching_impl: std::false_type{};
+struct hash_is_avalanching_impl:std::false_type{};
+
+template<typename IsAvalanching>
+struct avalanching_value
+{
+  static constexpr bool value=IsAvalanching::value;
+};
+
+/* may be explicitly marked as BOOST_DEPRECATED in the future */
+template<> struct avalanching_value<void>
+{
+  static constexpr bool value=true;
+};
 
 template<typename Hash>
-struct hash_is_avalanching_impl<Hash,
-  boost::unordered::detail::void_t<typename Hash::is_avalanching> >:
-    std::true_type{};
+struct hash_is_avalanching_impl<
+  Hash,
+  boost::unordered::detail::void_t<typename Hash::is_avalanching>
+>:std::integral_constant<
+  bool,
+  avalanching_value<typename Hash::is_avalanching>::value
+>{};
 
 } /* namespace detail */
 
@@ -32,8 +48,12 @@ struct hash_is_avalanching_impl<Hash,
  * when actual characterization differs from default.
  */
 
-/* hash_is_avalanching<Hash>::value is true when the type Hash::is_avalanching
- * is present, false otherwise.
+/* hash_is_avalanching<Hash>::value is:
+ *   - false if Hash::is_avalanching is not present.
+ *   - Hash::is_avalanching::value if this is present and constexpr-convertible
+ *     to a bool.
+ *   - true if Hash::is_avalanching is void (deprecated).
+ * UB otherwise.
  */
 template<typename Hash>
 struct hash_is_avalanching: detail::hash_is_avalanching_impl<Hash>::type{};