Re: [GHC] #15817: Visible kind application + data family = GHC panic, ‘impossible’ happened

27 Oct 2018

      #15817: Visible kind application + data family = GHC panic, ‘impossible’ happened
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      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:                    |
-------------------------------------+-------------------------------------
Changes (by Iceland_jack):

 * keywords:  TypeApplications => TypeInType
 * cc: mnguyen (removed)

Comment:

 Minimized example that compiles fine on GHC 8.6.1 but fails on HEAD

 {{{#!hs
 {-# Language RankNTypes       #-}
 {-# Language PolyKinds        #-}
 {-# Language TypeFamilies     #-}

 data family   X :: forall (a :: *). *
 data instance X = MkX
 }}}
 {{{
 $ ghci-stage2 -ignore-dot-ghci --interactive ~/hs/593_bug.hs
 GHCi, version 8.7.20181025: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /home/baldur/hs/593_bug.hs,
 interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20181025 for x86_64-unknown-linux):
         ASSERT failed!
   kind axiom D:R:Xa0 ::
          X = R:Xa -- Defined at /home/baldur/hs/593_bug.hs:6:15
   forall a. *
   * -> *
   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/FamInst.hs:158:61 in
 ghc:FamInst

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

 so it does not actually depend on VKA.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15817#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler