[GHC] #15076: Typed hole causes GHC to panic (No skolem info)

[GHC]

#15076: Typed hole causes GHC to panic (No skolem info)
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.2.2
  (Type checker)                     |
           Keywords:  TypedHoles,    |  Operating System:  Unknown/Multiple
  TypeInType                         |
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #14040
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Spun out from https://ghc.haskell.org/trac/ghc/ticket/14040#comment:2,
 which was different enough from the original program in #14040 to warrant
 its own ticket. The following program panics on every version of GHC from
 8.0.2 onward:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeInType #-}
 module Bug where

 import Data.Kind
 import Data.Proxy

 newtype S (f :: k1 -> k2)
   = MkS (forall t. Proxy t -> Proxy (f t))

 foo :: forall (a :: Type)
               (f :: forall (x :: a). Proxy x -> Type).
        S f -> ()
 foo (MkS (sF :: _)) = ()
 }}}
 {{{
 $ ~/Software/ghc/inplace/bin/ghc-stage2 --interactive Bug.hs
 GHCi, version 8.5.20180420: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:15:17: error:ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.5.20180420 for x86_64-unknown-linux):
         No skolem info:
   [a1_a1Cb[sk:1]]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1162:37 in
 ghc:Outputable
         pprPanic, called at compiler/typecheck/TcErrors.hs:3224:5 in
 ghc:TcErrors
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15076>
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