Compiling cabal with GHC HEAD

[Oleg Grenrus]

I tried to fix compilation of Cabal using Cabal HEAD. It’s trivial patch:

https://github.com/phadej/cabal/commit/525e0680505c74f42a321e55b357a27222790628

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?
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