[GHC] #11558: Using Cabal 1.22 against GHC 8.0 results in unhelpful errors

[GHC]

#11558: Using Cabal 1.22 against GHC 8.0 results in unhelpful errors
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Package system    |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by bgamari:

@@ -23,0 +23,10 @@
+ Finally, you need to clear your package database.
+
+ This can be done with,
+ {{{
+ $ git clone git://github.com/haskell/cabal
+ $ cd cabal
+ $ cabal update
+ $ cabal install Cabal/ cabal-install/
+ $ rm -R $HOME/.ghc/*-8.0.0*
+ }}}

New description:

 GHC 8.0 will require Cabal >1.22 due to a variety of interface changes
 (see #10714, Phab:D1780, https://mail.haskell.org/pipermail/ghc-
 devs/2016-January/010978.html). Unfortunately things currently blow up
 late in compilation with a seemingly unrelated error when this requirement
 isn't met. Namely, we complain about missing interface files for
 dependencies. For instance,

 {{{
 Text/Parsec/Prim.hs:85:1: error:
     Bad interface file: /root/.cabal/lib/x86_64-linux-
 ghc-8.0.0.20160204/mtl-2.2.1-9wMhzcfsAXi7cNtl9n7svO/Control/Monad/Cont/Class.hi
         Something is amiss; requested module
 mtl-2.2.1 at mtl-2.2.1-1af5dabd2d7abb688f2145aec87badc4:Control.Monad.Cont.Class
 differs from name found in the interface file
 mtl_9wMhzcfsAXi7cNtl9n7svO:Control.Monad.Cont.Class
 }}}

 = For users seeing this error =

 The solution here is to install `Cabal` 1.23 or later and a `cabal-
 install` linking against it. Currently this requires installing `Cabal`
 and `cabal-install` from git. This is most easily done using GHC 7.10.
 Finally, you need to clear your package database.

 This can be done with,
 {{{
 $ git clone git://github.com/haskell/cabal
 $ cd cabal
 $ cabal update
 $ cabal install Cabal/ cabal-install/
 $ rm -R $HOME/.ghc/*-8.0.0*
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11558#comment:4>
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