...assuming they were implicitly made private by accident rather than by design.
(And private derivation can cause unexpected failure of dynamic_cast, cf.
63b67ab5cab8cf7576a68cabe5fb1a42c6ad800c "Use public derivation, and remove
then-unnecessary downcasts.")
Change-Id: Id821afba34fd2f155e30fac903567707e46d1fde