[GHC] #15918: GHC panic from TypeApplications(?)

19 Nov 2018

      #15918: GHC panic from TypeApplications(?)
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
  TypeApplications                   |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Minimized from
 https://gist.github.com/Icelandjack/683bd4b79027695ffc31632645c9d58b, I
 don't expect `Build []` to kind check but ti shouldn't crash.

 {{{#!hs
 {-# Language PolyKinds             #-}
 {-# Language TypeFamilies          #-}
 {-# Language ConstraintKinds       #-}
 {-# Language FlexibleContexts      #-}
 {-# Language QuantifiedConstraints #-}
 {-# Language UndecidableInstances  #-}

 import Data.Kind

 class Rev f where
   rev :: f a

 instance (forall xx. cls xx => Rev xx) => Rev (Build cls) where
   rev = undefined

 data Build :: ((k -> Type) -> Constraint) -> (k -> Type)

 uu = rev :: Build [] a
 }}}

 gives a panic

 {{{
 $ ghc-stage2 --interactive -ignore-dot-ghci ~/hs/711_bug.hs
 GHCi, version 8.7.20181029: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( ~/hs/711_bug.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20181029 for x86_64-unknown-linux):
         ASSERT failed!
   irred_a1zW :: [(xx_a1zV[sk:3] |> Sym {co_a1zu})]
   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/TcType.hs:1826:53 in
 ghc:TcType

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

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15918
GHC http://www.haskell.org/ghc/
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