[GHC] #15859: Dependent quantification, GHC panic

[GHC]

#15859: Dependent quantification, GHC panic
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:  TypeInType     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 While exploring
 [https://gist.github.com/RyanGlScott/ded669b5ae3db1ce38c4b6021f144776
 Ryan's gist for term-level] `f :: forall a -> a -> Type`.

 {{{#!hs
 {-# Language PolyKinds          #-}
 {-# Language TypeApplications   #-}
 {-# Language ImpredicativeTypes #-}

 import Data.Kind

 data A k :: k -> Type

 f :: KindOf A
 f a = undefined

 type KindOf (a :: k) = k

 a = f @Int
 }}}

 gives

 {{{
 $ ~/code/unmodifiedghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-
 ghci 630_bug.hs
 GHCi, version 8.7.20181029: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( 630_bug.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20181029 for x86_64-unknown-linux):
         ASSERT failed!
   KindOf A
   forall k -> k -> *
   k_a1xQ[sk:0]
   k_a1xQ[sk:0] -> *
   k_a1xQ[sk:0]
   [req]
   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/TcExpr.hs:1336:94 in
 ghc:TcExpr

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

 >
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15859>
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