[tsimionescu]

I think this is only true upto some point. Ultimately the API of a unit of code is not fully defined by the public VS private language features, it is defined by the conventions for its use. If a field/method is public but it is documented to not be used by anything except, say, some tests, then it shouldn't be considered part of the actual API.

Even in languages which have a very powerful type system, there are assumptions that have to be left to documentation (e.g. that the Monad laws are respected by types which match the Monad typeclasses in Haskell). Testing parts which are documented to not be relevant is often actively harmful, since it causes problems with later changes.