From c14d3a1e2b2061394dd1b8cd743358e10cb0f300 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Sun, 3 Jul 2022 21:08:14 +0300
Subject: [PATCH] Add a comment explaining why to_underlying isn't used in
 hash_value for enums

---
 include/boost/container_hash/hash.hpp | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/boost/container_hash/hash.hpp b/include/boost/container_hash/hash.hpp
index b4066b1..f58c9c8 100644
--- a/include/boost/container_hash/hash.hpp
+++ b/include/boost/container_hash/hash.hpp
@@ -163,6 +163,20 @@ namespace boost
     typename boost::enable_if_<boost::is_enum<T>::value, std::size_t>::type
         hash_value( T v )
     {
+        // This should in principle return the equivalent of
+        //
+        // boost::hash_value( to_underlying(v) );
+        //
+        // However, the C++03 implementation of underlying_type,
+        //
+        // conditional<is_signed<T>, make_signed<T>, make_unsigned<T>>::type::type
+        //
+        // generates a legitimate -Wconversion warning in is_signed,
+        // because -1 is not a valid enum value when all the enumerators
+        // are nonnegative.
+        //
+        // So the legacy implementation will have to do for now.
+
         return static_cast<std::size_t>( v );
     }