[GHC] #15076: Typed hole with higher-rank kind causes GHC to panic (No skolem info)

[GHC]

#15076: Typed hole with higher-rank kind causes GHC to panic (No skolem info)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |             Keywords:  TypedHoles,
      Resolution:                    |  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14040, #14880    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 If you don't use a typed hole:

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

 import Data.Kind
 import Data.Proxy

 foo :: forall (a :: Type)
               (f :: forall (x :: a). Proxy x -> Type).
        Proxy f -> ()
 foo _ = ()
 }}}

 Then it compiles. However, it fails with a Core Lint error:

 {{{
 $ /opt/ghc/8.4.2/bin/ghc Bug.hs -dcore-lint
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 *** Core Lint errors : in result of Desugar (after optimization) ***
 <no location info>: warning:
     In the type ‘forall (x :: a) a (f :: forall (x :: a).
                                          Proxy x -> *).
                  Proxy (f x) -> ()’
     @ a_aZi[sk:1] is out of scope
 *** Offending Program ***
 foo
   :: forall (x :: a) a (f :: forall (x :: a). Proxy x -> *).
      Proxy (f x) -> ()
 [LclIdX]
 foo
   = \ (@ (x_a1e5 :: a_aZi[sk:1]))
       (@ a_a1e6)
       (@ (f_a1e7 :: forall (x :: a). Proxy x -> *))
       _ [Occ=Dead] ->
       ()

 $trModule :: Module
 [LclIdX]
 $trModule = Module (TrNameS "main"#) (TrNameS "Bug"#)

 *** End of Offense ***


 <no location info>: error:
 Compilation had errors
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15076#comment:5>
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