Rework error_code for better std interop

test/std_interop_test3.cpp

@@ -0,0 +1,63 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/error_code.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <cerrno>
+
+#if !defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+BOOST_PRAGMA_MESSAGE( "BOOST_SYSTEM_HAS_SYSTEM_ERROR not defined, test will be skipped" )
+int main() {}
+
+#else
+
+#include <system_error>
+
+int main()
+{
+    {
+        std::error_code e1;
+        boost::system::error_code e2 = e1;
+        std::error_code e3 = e2;
+
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        std::error_code e1( 5, std::system_category() );
+        boost::system::error_code e2 = e1;
+        std::error_code e3 = e2;
+
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        std::error_code e1( 0, std::generic_category() );
+        boost::system::error_code e2 = e1;
+        std::error_code e3 = e2;
+
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        std::error_code e1( ENOENT, std::generic_category() );
+        boost::system::error_code e2 = e1;
+        std::error_code e3 = e2;
+
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        boost::system::error_code e2 = make_error_code( std::errc::no_such_file_or_directory );
+        std::error_code e3 = e2;
+
+        BOOST_TEST_EQ( make_error_code( std::errc::no_such_file_or_directory ), e3 );
+    }
+
+    return boost::report_errors();
+}
+
+#endif