[GHC] #16245: GHC panic (No skolem info) with QuantifiedConstraints and strange scoping

27 Jan 2019

      #16245: GHC panic (No skolem info) with QuantifiedConstraints and strange scoping
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
  (Type checker)                     |
           Keywords:  TypeInType,    |  Operating System:  Unknown/Multiple
  QuantifiedConstraints              |
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following program panics with GHC 8.6.3 and HEAD:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE QuantifiedConstraints #-}
 module Bug where

 import Data.Kind

 type Const a b = a
 type SameKind (a :: k) (b :: k) = (() :: Constraint)
 class (forall (b :: k). SameKind a b) => C (k :: Const Type a)
 }}}
 {{{
 $ /opt/ghc/8.6.3/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:11:36: error:ghc: panic! (the 'impossible' happened)
   (GHC version 8.6.3 for x86_64-unknown-linux):
         No skolem info:
   [k1_a1X4[sk:1]]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/typecheck/TcErrors.hs:2891:5 in
 ghc:TcErrors
 }}}

 As with #16244, I imagine that the real culprit is that `SameKind a b`
 would force `a :: k`, which would be ill-scoped.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16245
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