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{-# 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.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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