[ 2016-06, Oulu — Jonathan comments and provides wording ]

The sentence "The signature with argument ec returns false if an error occurs." means that given a file such that status(p).type() == file_type::unknown, exists(p) is true but exists(p, ec) is false.

I believe we should make the behaviour of exists(p) and exists(p, ec) consistent, so that the latter clears ec except when the former would throw an exception, which is only for the file_type::none case.