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Show instance for Digraph.Forest overlaps with Show [a]

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.
Trac metadata
Trac field Value
Version 7.4.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component GHC API
Test case
Differential revisions
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Blocking
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Operating system
Architecture
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