[GHC] #15862: Typeable panic with promoted rank-2 kind (initDs)

[GHC]

#15862: Typeable panic with promoted rank-2 kind (initDs)
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:  Typeable       |  Operating System:  Unknown/Multiple
       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 on GHC 8.2 and later:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE ImpredicativeTypes #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeApplications #-}
 module Bug where

 import Type.Reflection

 newtype Foo = MkFoo (forall a. a)

 foo :: TypeRep MkFoo
 foo = typeRep @MkFoo
 }}}
 {{{
 $ /opt/ghc/8.6.1/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 GHC error in desugarer lookup in Bug:
   attempting to use module ‘main:Bug’ (Bug.hs) which is not loaded
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.6.1 for x86_64-unknown-linux):
         initDs
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15862>
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