If the return type if fixed for each implementation of the interface,
you should add the type parameter to the interface:
Also note that the above type parameter is the correct declaration of a "type that is comparable to itself". Your declaration was just a "type that is comparable to a (potentially different) comparable type", as far as I can tell.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus