[GHC] #15796: Core Lint error with visible kind application

[GHC]

#15796: Core Lint error with visible kind application
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12045            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Yes it is Simon this uses [https://phabricator.haskell.org/D5229 D5229
 (Visible kind application)]. Here is another program that triggers a
 similar error

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

 {-# Options_GHC -dcore-lint #-}

 import qualified GHC.TypeLits as TypeLits
 import Data.Kind

 data OP a = Op a

 type family
  UnOp (op_a :: OP a) :: a where
  UnOp (Op a) = a

 class Ríki (obj :: Type) where
  type (-->) :: OP obj -> obj -> Type

 data (<=) :: OP TypeLits.Nat -> TypeLits.Nat -> Type where
   LessThan :: TypeLits.KnownNat (UnOp op_a)
            => op_a <= b

 instance Ríki TypeLits.Nat where
  type (-->) = (<=)
 }}}

 {{{
 $ ghci -ignore-dot-ghci 572_bug.hs
 GHCi, version 8.7.20181017: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( 572_bug.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20181017 for x86_64-unknown-linux):
         Core Lint error
   <no location info>: warning:
       In the type ‘(<=)’
       Found TcTyCon: <=[tc]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/typecheck/FamInst.hs:171:31 in
 ghc:FamInst

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 >
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15796#comment:3>
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