Add constexpr to virtual functions on C++20 or later. Refs #141.

test/failed_constexpr_test2.cpp

@@ -0,0 +1,54 @@
+// Copyright 2026 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/error_code.hpp>
+#include <boost/system/error_condition.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <boost/static_assert.hpp>
+#include <cstdio>
+
+#if !defined(BOOST_SYSTEM_HAS_CXX20_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr test, BOOST_SYSTEM_HAS_CXX20_CONSTEXPR isn't defined")
+int main() {}
+
+#else
+
+namespace sys = boost::system;
+
+class user_category: public sys::error_category
+{
+public:
+
+    constexpr virtual const char* name() const noexcept
+    {
+        return "user";
+    }
+
+    virtual std::string message( int ev ) const
+    {
+        char buffer[ 256 ];
+        std::snprintf( buffer, sizeof( buffer ), "user message %d", ev );
+
+        return buffer;
+    }
+};
+
+static constexpr user_category s_user_cat;
+
+constexpr sys::error_code ec( 1, s_user_cat );
+
+BOOST_STATIC_ASSERT( ec.failed() );
+BOOST_STATIC_ASSERT( ec );
+BOOST_STATIC_ASSERT( !!ec );
+
+constexpr sys::error_condition en( 1, s_user_cat );
+
+BOOST_STATIC_ASSERT( en.failed() );
+BOOST_STATIC_ASSERT( en );
+BOOST_STATIC_ASSERT( !!en );
+
+int main() {}
+
+#endif