Oh, this is irritating. The problem is we recently updated Cabal the library to get rid of InstalledPackageInfo_ (so there is only InstalledPackageInfo now) but it looks like in some situations cabal-install can be compiled with an old version of Cabal (as is happening to you). I suppose an appropriate remedy is to bump the Cabal library dependency in cabal-install so we don't attempt to use the old Cabal; alternately we could preprocessor macro to make it work in both cases. Edward Excerpts from Oleg Grenrus's message of 2015-08-19 07:46:56 -0700:
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?