Part of the problem here is that your new could be improved.

Well, I was trying to keep the example simple :-)

After all, a later subclass could take as its logger argument either a Logger or a logspec, which can be used to build an appropriate logger. All of a sudden your assertion about the logger argument is incorrect.

In which case, I would change it.

My aim when I do this sort of thing is to make the assumptions (or contracts if you prefer) of my classes to be explicit. Where possible I like to make them explicit using code rather than comments or documentation.

If my contracts are wrong, I fix them. Having them in code makes it very obvious when they're wrong - which is a plus in my book :-)