GHC rejects use of mulit-param type class.

In the following example, I instantiate a few small typeclasses. The last instantiation provided does not compile, despite it being virtually identical the prior instantiation.

{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}

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class Collection collection handle | collection -> handle, handle -> collection where
    getHandles :: collection -> [handle]

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data HandleX = HandleX Int
data CollectionX = CollectionX [HandleX]

instance Collection CollectionX HandleX where
    getHandles (CollectionX handles) = handles

--------------------------------------------------------------------------------

newtype HandleY = HandleY HandleX
newtype CollectionY = CollectionY CollectionX

instance Collection CollectionY HandleY where
    getHandles (CollectionY collectionX) = map HandleY $ getHandles collectionX
    -- Good, this compiles!

--------------------------------------------------------------------------------

newtype HandleZ = HandleZ HandleY
newtype CollectionZ = CollectionZ CollectionY

instance Collection CollectionZ HandleZ where
    getHandle (CollectionZ collectionY) = map HandleZ $ getHandles collectionY
    -- Error: `getHandle' is not a (visible) method of class `Collection'

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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