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Consider the following code:

{-# LANGUAGE TypeFamilies, PolyKinds, MultiParamTypeClasses,
             FlexibleInstances, DataKinds #-}

data KProxy (a :: *) = KP

class Foo (a :: k1) (b :: k2) where
  type Bar a

instance Foo (a :: k1) (b :: k2) where
  type Bar a = (KP :: KProxy k2)

The kind variable k2 appears on the RHS of the instance for Bar but not on the left-hand side. This is disallowed for type variables, but there seems to be no check for kind variables.

I will fix in an upcoming patch.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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