Core Lint error in Specialise with PolyKinds and derived instance

The following causes a lint error when compiled with -O (but not with -O0):

{-# LANGUAGE PolyKinds #-}
{-# OPTIONS_GHC -dcore-lint -O #-}

data T a b = MkT (a b)
  deriving Show

main = print (MkT (Just True))

For reference, the beginning of the (rather long) error message is:

*** Core Lint errors : in result of Specialise ***
<no location info>: Warning:
    In the type ‛Main.T k_XlN Data.Maybe.Maybe GHC.Types.Bool’
    Kind application error in
      type ‛Main.T k_XlN Data.Maybe.Maybe GHC.Types.Bool’
      Function kind = forall (k_alL :: BOX). (k_alL -> *) -> k_alL -> *
      Arg kinds = [(k_XlN, BOX), (Data.Maybe.Maybe, * -> *),
                   (GHC.Types.Bool, *)]

It looks like something is wrong with the types of the derived Show instance, when PolyKinds is enabled. A similar problem applies to Eq (and perhaps other classes).

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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