[GHC] #5960: Show instance for Digraph.Forest overlaps with Show [a]

[GHC]

#5960: Show instance for Digraph.Forest overlaps with Show [a]
------------------------------+---------------------------------------------
 Reporter:  SimonHengel       |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Component:  GHC API         
  Version:  7.4.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
------------------------------+---------------------------------------------
 Steps to reproduce:

 {{{
 module Foo where

 import Digraph ()

 newtype MyList a = MyList [a]
   deriving Show
 }}}

 {{{
 $ ghci -packageghc Foo.hs
 Foo.hs:6:12:
     Overlapping instances for Show [a]
       arising from the 'deriving' clause of a data type declaration
     Matching instances:
       instance Show a => Show [a] -- Defined in `GHC.Show'
       instance Show a => Show (Digraph.Forest a) -- Defined in `Digraph'
     (The choice depends on the instantiation of `a'
      To pick the first instance above, use -XIncoherentInstances
      when compiling the other instance declarations)
     When deriving the instance for (Show (MyList a))
 Failed, modules loaded: none.
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5960>
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