Doesn't accept type

import Data.Kind
import qualified Control.Category as Cat

newtype Transformer f g where
  Transform :: (forall i. f i ~> g i) -> Transformer f g

type family 
  (~>) :: k -> k -> Type where
  (~>) = (->)
  (~>) = Transformer
 
instance 
  Cat.Category ((~>) :: k -> k -> Type) 
  => 
  Cat.Category (Transformer :: (i -> k) -> (i -> k) -> Type) where

  id :: forall (f :: i -> k). Transformer f f
  id = Transform Cat.id

works, as well as omitting the instance signature:

  id = Transform Cat.id

Implicitly quantifying f or omitting its kind signature result in the same error:

  id :: Transformer f f 
  id = Transform Cat.id

--     • Could not deduce (Cat.Category (~>))
--         arising from a use of ‘Cat.id’
--       from the context: Cat.Category (~>)
--         bound by the instance declaration at /tmp/tX81.hs:(13,3)-(15,60)
--     • In the first argument of ‘Transform’, namely ‘Cat.id’
--       In the expression: Transform Cat.id
--       In an equation for ‘id’: id = Transform Cat.id
-- Compilation failed.

Should it compile

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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