SQ has a codeshare agreement with LO, but it appears that you can only book these tickets/routes online on the LO site. You cannot even select WAW as an airport on the SQ website.
Is this just being slack on SQ's part?
Even at Expedia you can purchase such a ticket, where it lists "Singapore Airlines 346 / Singapore Airlines 2312 (2312 operated by LOT-Polish Airlines)."
Seems strange for SQ to have such codeshare flights... while at the same time their website is the only place... NOT to sell them. I'm sure LO isn't complaining though, and neither is LH...