Add test that checks for duplicate category instances

test/single_instance_test.cpp

@@ -0,0 +1,32 @@
+
+// Copyright 2018 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+
+#include <boost/system/error_code.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost::system;
+
+namespace lib1
+{
+
+error_code get_system_code();
+error_code get_generic_code();
+
+} // namespace lib1
+
+namespace lib2
+{
+
+error_code get_system_code();
+error_code get_generic_code();
+
+} // namespace lib2
+
+int main()
+{
+    BOOST_TEST_EQ( lib1::get_system_code(), lib2::get_system_code() );
+    BOOST_TEST_EQ( lib1::get_generic_code(), lib2::get_generic_code() );
+
+    return boost::report_errors();
+}