Update C++03 deprecation message

include/boost/system/detail/mutex.hpp

@@ -31,7 +31,9 @@ struct mutex
 } // namespace system
 } // namespace boost
 
-#elif defined(BOOST_MSSTL_VERSION) && BOOST_MSSTL_VERSION >= 140
+#else // defined(BOOST_SYSTEM_DISABLE_THREADS)
+
+#if defined(BOOST_MSSTL_VERSION) && BOOST_MSSTL_VERSION >= 140
 
 // Under the MS STL, std::mutex::mutex() is not constexpr, as is
 // required by the standard, which leads to initialization order
@@ -40,6 +42,14 @@ struct mutex
 
 #include <shared_mutex>
 
+#if BOOST_MSSTL_VERSION >= 142 || _HAS_SHARED_MUTEX
+# define BOOST_SYSTEM_HAS_MSSTL_SHARED_MUTEX
+#endif
+
+#endif
+
+#if defined(BOOST_SYSTEM_HAS_MSSTL_SHARED_MUTEX)
+
 namespace boost
 {
 namespace system
@@ -53,7 +63,7 @@ typedef std::shared_mutex mutex;
 } // namespace system
 } // namespace boost
 
-#else
+#else // defined(BOOST_SYSTEM_HAS_MSSTL_SHARED_MUTEX)
 
 #include <mutex>
 
@@ -70,7 +80,8 @@ using std::mutex;
 } // namespace system
 } // namespace boost
 
-#endif
+#endif // defined(BOOST_SYSTEM_HAS_MSSTL_SHARED_MUTEX)
+#endif // defined(BOOST_SYSTEM_DISABLE_THREADS)
 
 namespace boost
 {

include/boost/system/detail/requires_cxx11.hpp

@@ -14,7 +14,7 @@
     defined(BOOST_NO_CXX11_DEFAULTED_FUNCTIONS) || \
     defined(BOOST_NO_CXX11_HDR_SYSTEM_ERROR)
 
-BOOST_PRAGMA_MESSAGE("C++03 support is deprecated in Boost.System 1.82 and will be removed in Boost.System 1.84.")
+BOOST_PRAGMA_MESSAGE("C++03 support was deprecated in Boost.System 1.82 and will be removed in Boost.System 1.84. Please open an issue in https://github.com/boostorg/system if you want it retained.")
 
 #endif