Cannot derive well-kinded instance of form ‘Category

Hello everyone, sorry if this was already reported somewhere.

I'm playing with GHC 7.8.1 RC2 and I'm updating hsenv to support GHC 7.8. This is a real code snippet, which was compiling fine in GHC 7.6.x

newtype ArgArrow a b = ArgArrow (StaticArrowT KnownArgs (Kleisli (ReaderT Args IO)) a b)
    deriving (Category, Arrow, ArrowChoice)

but that yields the following in GHC 7.8.1-RC2

 Cannot derive well-kinded instance of form ‘Category (ArgArrow ...)’
      Class ‘Category’ expects an argument of kind ‘k -> k -> *’
    In the newtype declaration for ‘ArgArrow’

This might be related to the new feature of GHC 7.8, namely "kind variables" (you get the idea, even if the name is not 100% accurate). To make the code compile I had to enable StandaloneDeriving and write the following:

newtype ArgArrow a b = ArgArrow (StaticArrowT KnownArgs (Kleisli (ReaderT Args IO)) a b)
    deriving (Arrow, ArrowChoice)

deriving instance Category ArgArrow

Is this by design or is a genuine bug? Thanks!

Alfredo

Trac metadata

Trac field	Value
Version	7.8.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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