[GHC] #13684: Core lint error with defer-typed-holes and coerce

10 May 2017

      #13684: Core lint error with defer-typed-holes and coerce
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           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# Options_GHC -dcore-lint -fdefer-typed-holes #-}

 {-# Language FlexibleInstances, UndecidableInstances,
 MultiParamTypeClasses, GeneralizedNewtypeDeriving #-}

 import Data.Traversable
 import Control.Lens.Indexed
 import Control.Lens.Fold   (ifoldMapOf)
 import Control.Lens.Setter (iover)

 newtype Foo a = F (WrapITraversable [] a)

 newtype WrapITraversable f a = WIT (f a)

 instance TraversableWithIndex i f => Functor (WrapITraversable f) where
   fmap = fmapDefault

 instance TraversableWithIndex i f => Foldable (WrapITraversable f) where
   foldMap = foldMapDefault

 instance TraversableWithIndex i f => Traversable (WrapITraversable f)
 where
   traverse = itraverse . const

 instance TraversableWithIndex i f => FoldableWithIndex i (WrapITraversable
 f) where
   ifoldMap = ifoldMapOf itraversed

 instance TraversableWithIndex i f => FunctorWithIndex i (WrapITraversable
 f) where
   imap = iover itraversed

 instance TraversableWithIndex i f => TraversableWithIndex i
 (WrapITraversable f) where
   itraverse = coerce itraverse
 }}}

 causes with `lens 4.14` or `4.15.1`, GHC 8.0.1.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13684
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