Extract is_error_code_enum, is_error_condition_enum into their own headers

test/is_error_code_enum_test.cpp

@@ -0,0 +1,41 @@
+// Copyright 2020 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/is_error_code_enum.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+enum error
+{
+    success = 0,
+    e1,
+    e2,
+    e3
+};
+
+namespace boost
+{
+namespace system
+{
+
+template<> struct is_error_code_enum< ::error >
+{
+    static const bool value = true;
+};
+
+}
+}
+
+boost::system::error_code make_error_code( ::error e );
+
+enum not_error
+{
+};
+
+int main()
+{
+    BOOST_TEST( boost::system::is_error_code_enum< ::error >::value );
+    BOOST_TEST( !boost::system::is_error_code_enum< ::not_error >::value );
+
+    return boost::report_errors();
+}