I tried to fix compilation of Cabal using Cabal HEAD. It’s trivial patch: https://github.com/phadej/cabal/commit/525e0680505c74f42a321e55b357a27222790... but it breaks build on every other released GHC: https://travis-ci.org/phadej/cabal/builds/76288656 — The original issue GHC-7.11 complained was: Distribution/Client/Types.hs:71:10: error: Illegal instance declaration for ‘PackageFixedDeps InstalledPackageInfo’ (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use TypeSynonymInstances if you want to disable this.) In the instance declaration for ‘PackageFixedDeps InstalledPackageInfo’ So I had to add TypeSynonymInstances and FlexibleInstances And also had to change import of InstalledPackageInfo(exposed) in Haddock module. — At this point I’m really confused. I cannot find ‘InstalledPackageInfo_’ symbol anywhere. Can someone explain what happens?