[GHC] #15807: GHC panic with visible kind applications

[GHC]

#15807: GHC panic with visible kind applications
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           Reporter:  Iceland_jack      |             Owner:  (none)
               Type:  bug               |            Status:  new
           Priority:  normal            |         Milestone:
          Component:  Compiler          |           Version:  8.6.1
           Keywords:  TypeApplications  |  Operating System:  Linux
       Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
          Test Case:                    |        Blocked By:
           Blocking:                    |   Related Tickets:
Differential Rev(s):                    |         Wiki Page:
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 Using the https://phabricator.haskell.org/D5229 diff

 This is '''fine'''

 {{{#!hs
 {-# Language RankNTypes       #-}
 {-# Language TypeApplications #-}
 {-# Language PolyKinds        #-}
 {-# Language GADTs            #-}

 import Data.Kind

 data
   App :: forall (f :: Type -> Type). Type -> Type
   where
   MkApp :: f a -> App @f a
 }}}

 Kind polymorphic is '''fine'''

 {{{#!hs
 {-# Language RankNTypes       #-}
 {-# Language TypeApplications #-}
 {-# Language PolyKinds        #-}
 {-# Language GADTs            #-}

 import Data.Kind

 data
   App :: forall k (f :: k -> Type). k -> Type
   where
   MkApp :: f a -> App @k @(f :: k -> Type) (a :: k)
 }}}

 But offing the visibility of `k` '''fails'''

 {{{#!hs
 {-# Language RankNTypes       #-}
 {-# Language TypeApplications #-}
 {-# Language PolyKinds        #-}
 {-# Language GADTs            #-}

 import Data.Kind

 data
   App :: forall (f :: k -> Type). k -> Type
   where
   MkApp :: f a -> App @(f :: k -> Type) (a :: k)
 }}}

 {{{#!hs
 {-# Language RankNTypes       #-}
 {-# Language TypeApplications #-}
 {-# Language PolyKinds        #-}
 {-# Language GADTs            #-}

 import Data.Kind

 data
   App :: forall (f :: k -> Type). k -> Type
   where
   MkApp :: f a -> App @f a
 }}}

 {{{
 $ ghci -ignore-dot-ghci 581_bug.hs
 GHCi, version 8.7.20181017: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( 581_bug.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20181017 for x86_64-unknown-linux):
         ASSERT failed!
   2
   1
   f_a1yB[tau:1]
   f_a1yx[sk:1]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/utils/Outputable.hs:1219:5 in
 ghc:Outputable
         assertPprPanic, called at compiler/typecheck/TcMType.hs:778:54 in
 ghc:TcMType

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

 >
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15807>
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