From 5f43b228612f45a68705d691b482aeca7cf3f68c Mon Sep 17 00:00:00 2001
From: Andrey Semashev <andrey.semashev@gmail.com>
Date: Sat, 18 Feb 2023 19:58:36 +0300
Subject: [PATCH] Reuse is_swappable implementation in is_nothrow_swappable for
 gcc < 4.7.

This avoids applying noexcept operator to a potentially invalid
swap expression, which should resolve ICE with gcc 4.6.
---
 doc/history.qbk                               |  1 +
 .../detail/is_swappable_cxx_11.hpp            | 21 +++++++++++++++++--
 .../type_traits/is_nothrow_swappable.hpp      |  3 +--
 test/is_nothrow_swappable_test.cpp            |  1 -
 4 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/doc/history.qbk b/doc/history.qbk
index d2b277f..ca412ba 100644
--- a/doc/history.qbk
+++ b/doc/history.qbk
@@ -10,6 +10,7 @@
 [h4 Boost-1.82.0]
 
 * Added __is_swappable trait.
+* Added a workaround for gcc 4.6 that allows __is_nothrow_swappable to work.
 
 [h4 Boost-1.70.0]
 
diff --git a/include/boost/type_traits/detail/is_swappable_cxx_11.hpp b/include/boost/type_traits/detail/is_swappable_cxx_11.hpp
index 293210a..390253c 100644
--- a/include/boost/type_traits/detail/is_swappable_cxx_11.hpp
+++ b/include/boost/type_traits/detail/is_swappable_cxx_11.hpp
@@ -34,7 +34,22 @@ template<class T> boost::false_type is_swappable_impl( ... );
 template<class T>
 struct is_swappable_helper { typedef decltype( boost_type_traits_swappable_detail::is_swappable_impl<T>(0) ) type; };
 
-#if !defined(BOOST_NO_CXX11_NOEXCEPT) && !BOOST_WORKAROUND(BOOST_GCC, < 40700)
+#if !defined(BOOST_NO_CXX11_NOEXCEPT)
+
+#if BOOST_WORKAROUND(BOOST_GCC, < 40700)
+
+// gcc 4.6 ICEs when noexcept operator is used on an invalid expression
+template<class T, class U, bool = is_swappable_with_helper<T, U>::type::value>
+struct is_nothrow_swappable_with_helper { typedef boost::false_type type; };
+template<class T, class U>
+struct is_nothrow_swappable_with_helper<T, U, true> { typedef boost::integral_constant<bool, noexcept(swap(boost::declval<T>(), boost::declval<U>()))> type; };
+
+template<class T, bool = is_swappable_helper<T>::type::value>
+struct is_nothrow_swappable_helper { typedef boost::false_type type; };
+template<class T>
+struct is_nothrow_swappable_helper<T, true> { typedef boost::integral_constant<bool, noexcept(swap(boost::declval<T&>(), boost::declval<T&>()))> type; };
+
+#else // BOOST_WORKAROUND(BOOST_GCC, < 40700)
 
 template<class T, class U, bool B = noexcept(swap(boost::declval<T>(), boost::declval<U>()))> boost::integral_constant<bool, B> is_nothrow_swappable_with_impl( int );
 template<class T, class U> boost::false_type is_nothrow_swappable_with_impl( ... );
@@ -46,7 +61,9 @@ template<class T> boost::false_type is_nothrow_swappable_impl( ... );
 template<class T>
 struct is_nothrow_swappable_helper { typedef decltype( boost_type_traits_swappable_detail::is_nothrow_swappable_impl<T>(0) ) type; };
 
-#endif // !defined(BOOST_NO_CXX11_NOEXCEPT) && !BOOST_WORKAROUND(BOOST_GCC, < 40700)
+#endif // BOOST_WORKAROUND(BOOST_GCC, < 40700)
+
+#endif // !defined(BOOST_NO_CXX11_NOEXCEPT)
 
 } // namespace boost_type_traits_swappable_detail
 
diff --git a/include/boost/type_traits/is_nothrow_swappable.hpp b/include/boost/type_traits/is_nothrow_swappable.hpp
index fc319f7..1ab3dcd 100644
--- a/include/boost/type_traits/is_nothrow_swappable.hpp
+++ b/include/boost/type_traits/is_nothrow_swappable.hpp
@@ -8,10 +8,9 @@
 //  http://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/config.hpp>
-#include <boost/config/workaround.hpp>
 
 #if defined(BOOST_NO_SFINAE_EXPR) || defined(BOOST_NO_CXX11_NOEXCEPT) || defined(BOOST_NO_CXX11_DECLTYPE) \
-   || defined(BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS) || BOOST_WORKAROUND(BOOST_GCC, < 40700)
+   || defined(BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS)
 
 #include <boost/type_traits/is_scalar.hpp>
 #include <boost/type_traits/is_const.hpp>
diff --git a/test/is_nothrow_swappable_test.cpp b/test/is_nothrow_swappable_test.cpp
index fafa526..ad49f95 100644
--- a/test/is_nothrow_swappable_test.cpp
+++ b/test/is_nothrow_swappable_test.cpp
@@ -18,7 +18,6 @@
 
 // These conditions should be similar to those in is_nothrow_swappable.hpp
 #if defined(BOOST_NO_SFINAE_EXPR) || defined(BOOST_NO_CXX11_NOEXCEPT) || defined(BOOST_NO_CXX11_DECLTYPE) || defined(BOOST_NO_CXX11_FUNCTION_TEMPLATE_DEFAULT_ARGS) \
-    || BOOST_WORKAROUND(BOOST_GCC, < 40700) \
     || (defined(__GLIBCXX__) && __GLIBCXX__ <= 20120301) // built-in clang++ -std=c++11 on Travis, w/ libstdc++ 4.6
 #define BOOST_TYPE_TRAITS_IS_NOTHROW_SWAPPABLE_EMULATED
 #endif