I agree with the solution in the second link: if you want a certain
condition to hold on the parameter, it needs to be made explicit. As for
whether this restriction is necessary, I believe it is because I don't see
any way of deciding the question 'a Monad.t = 'a u without examining the
abstract type hidden in a particular Logger.

What if you made LOGGER a functor parameterized on a Monad?