[GHC] #8196: Core Lint error in Specialise with PolyKinds and derived instance

[GHC]

#8196: Core Lint error in Specialise with PolyKinds and derived instance
-------------------------+-------------------------------------------------
       Reporter:         |             Owner:
  adamgundry             |            Status:  new
           Type:  bug    |         Milestone:
       Priority:         |           Version:  7.7
  normal                 |  Operating System:  Unknown/Multiple
      Component:         |   Type of failure:  Incorrect warning at
  Compiler               |  compile-time
       Keywords:         |         Test Case:
   Architecture:         |          Blocking:
  Unknown/Multiple       |
     Difficulty:         |
  Unknown                |
     Blocked By:         |
Related Tickets:         |
-------------------------+-------------------------------------------------
 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).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8196>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler