/usr/lib/ghc-doc/hoogle/libghc-dual-tree-doc.txt is in libghc-dual-tree-doc 0.2.0.9-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-dual-tree-doc/html/dual-tree.txt