/usr/lib/ghc-doc/hoogle/dependent-sum.txt is in libghc-dependent-sum-doc 0.2.0.1-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-dependent-sum-doc/html/dependent-sum.txt