[GHC] #15664: Core Lint error

[GHC]

#15664: Core Lint error
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Iceland_jack):

 Short version

 {{{#!hs
 {-# Language RankNTypes, TypeOperators, DataKinds, PolyKinds, GADTs,
 TypeInType, TypeFamilies #-}

 {-# Options_GHC -dcore-lint #-}

 import Data.Kind

 type family Apply (kind) (f :: kind) :: Type
 data        ApplyT(kind) :: kind -> Type

 type f ~> g = (forall xx. f xx -> g xx)

 unravel :: ApplyT(k) ~> Apply(k)
 unravel = unravel
 }}}

 {{{
 $ ghci -ignore-dot-ghci hs/443.hs
 GHCi, version 8.7.20180828: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( hs/443.hs, interpreted )
 *** Core Lint errors : in result of Desugar (before optimization) ***
 <no location info>: warning:
     In the type ‘forall k. ApplyT k ~> Apply k’
     Un-saturated type application Apply k_a1y2
 *** Offending Program ***
 Rec {
 $tcApplyT :: TyCon
 [LclIdX]
 $tcApplyT
   = TyCon
       14646326419187070856##
       770477529860249545##
       $trModule
       (TrNameS "ApplyT"#)
       1#
       $krep_a1Ad

 $krep_a1Ae [InlPrag=NOUSERINLINE[~]] :: KindRep
 [LclId]
 $krep_a1Ae = $WKindRepVar (I# 0#)

 $krep_a1Ad [InlPrag=NOUSERINLINE[~]] :: KindRep
 [LclId]
 $krep_a1Ad = KindRepFun $krep_a1Ae krep$*

 $trModule :: Module
 [LclIdX]
 $trModule = Module (TrNameS "main"#) (TrNameS "Main"#)

 unravel :: forall k. ApplyT k ~> Apply k
 [LclIdX]
 unravel
   = \ (@ k_a1zb) (@ (xx_a1zc :: k_a1zb)) ->
       break<0>() unravel @ k_a1zb @ xx_a1zc
 end Rec }

 *** End of Offense ***


 <no location info>: error:
 Compilation had errors


 *** Exception: ExitFailure 1
 >
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15664#comment:1>
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