From a9baab299fb37a4938d7fe5f00db19c7a3e0da53 Mon Sep 17 00:00:00 2001
From: Romain Geissler <romain.geissler@amadeus.com>
Date: Fri, 5 Sep 2025 23:31:26 +0000
Subject: [PATCH] Use underlying type for enums in is_signed/is_unsigned.

With clang >= 21, using is_signed/is_unsigned with "small" enums
(having an underlying type smaller than an int) results in strong error,
after being reported as warnings for years (see #171).

Since boost::is_signed/is_unsigned differs explicitly from the std one,
and can return "true" for enums (contrary to the std one), keep this
boost behavior and implement a special check for enums.

This allows in particular boost::lexical_cast to keep working when
converting non-scoped enums from int to strings with clang >= 21,
see boostorg/lexical_cast#87).
---
 include/boost/type_traits/is_signed.hpp   | 20 +++++++++++++++++---
 include/boost/type_traits/is_unsigned.hpp | 19 +++++++++++++++++--
 2 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/include/boost/type_traits/is_signed.hpp b/include/boost/type_traits/is_signed.hpp
index 4d50bf8c6f..2ba96492ab 100644
--- a/include/boost/type_traits/is_signed.hpp
+++ b/include/boost/type_traits/is_signed.hpp
@@ -13,6 +13,7 @@
 #include <boost/type_traits/is_integral.hpp>
 #include <boost/type_traits/remove_cv.hpp>
 #include <boost/type_traits/is_enum.hpp>
+#include <boost/core/underlying_type.hpp>
 #include <climits>
 
 namespace boost {
@@ -25,6 +26,18 @@ namespace boost {
 
 namespace detail{
 
+template <class no_cv_t, bool is_enum>
+struct is_signed_values_underlying_type
+{
+   typedef no_cv_t underlying_type;
+};
+
+template <class no_cv_t>
+struct is_signed_values_underlying_type<no_cv_t, true>
+{
+   typedef typename ::boost::underlying_type<no_cv_t>::type underlying_type;
+};
+
 template <class T>
 struct is_signed_values
 {
@@ -34,14 +47,15 @@ struct is_signed_values
    // the correct answer.
    //
    typedef typename remove_cv<T>::type no_cv_t;
-   static const no_cv_t minus_one = (static_cast<no_cv_t>(-1));
-   static const no_cv_t zero = (static_cast<no_cv_t>(0));
+   typedef typename is_signed_values_underlying_type<no_cv_t, ::boost::is_enum<T>::value>::underlying_type underlying_type;
+
+   static const underlying_type minus_one = (static_cast<underlying_type>(-1));
+   static const underlying_type zero = (static_cast<underlying_type>(0));
 };
 
 template <class T>
 struct is_signed_helper
 {
-   typedef typename remove_cv<T>::type no_cv_t;
    BOOST_STATIC_CONSTANT(bool, value = (!(::boost::detail::is_signed_values<T>::minus_one  > boost::detail::is_signed_values<T>::zero)));
 };
 
diff --git a/include/boost/type_traits/is_unsigned.hpp b/include/boost/type_traits/is_unsigned.hpp
index 38b916296d..34e4e16f13 100644
--- a/include/boost/type_traits/is_unsigned.hpp
+++ b/include/boost/type_traits/is_unsigned.hpp
@@ -13,6 +13,7 @@
 #include <boost/type_traits/is_integral.hpp>
 #include <boost/type_traits/is_enum.hpp>
 #include <boost/type_traits/remove_cv.hpp>
+#include <boost/core/underlying_type.hpp>
 
 #include <climits>
 
@@ -26,6 +27,18 @@ namespace boost {
 
 namespace detail{
 
+template <class no_cv_t, bool is_enum>
+struct is_unsigned_values_underlying_type
+{
+   typedef no_cv_t underlying_type;
+};
+
+template <class no_cv_t>
+struct is_unsigned_values_underlying_type<no_cv_t, true>
+{
+   typedef typename ::boost::underlying_type<no_cv_t>::type underlying_type;
+};
+
 template <class T>
 struct is_unsigned_values
 {
@@ -35,8 +48,10 @@ struct is_unsigned_values
    // the correct answer.
    //
    typedef typename remove_cv<T>::type no_cv_t;
-   static const no_cv_t minus_one = (static_cast<no_cv_t>(-1));
-   static const no_cv_t zero = (static_cast<no_cv_t>(0));
+   typedef typename is_unsigned_values_underlying_type<no_cv_t, ::boost::is_enum<T>::value>::underlying_type underlying_type;
+
+   static const underlying_type minus_one = (static_cast<underlying_type>(-1));
+   static const underlying_type zero = (static_cast<underlying_type>(0));
 };
 
 template <class T>