Compiling cabal with GHC HEAD

[Edward Z. Yang]

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/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?