It's here [link]
basically it's a co-op puzzle game, but the twist is that the other players are controlled by past and future versions of yourself. Combining that with portal would be crazy, you could have puzzles where you have to jump at just the right moment for your past self to fling you across the room, but they can't see you because you don't exist yet, or you could have to push a button to open a door, but it drops past you into a pit that you have to rescue them from to avoid a paradox, but if you don't spot the pit the first time you might not wait for a rescue and just act normally, making it difficult for future you to line up a portal in time...