[GHC] #14765: Levity polymorphism panic

[GHC]

#14765: Levity polymorphism panic
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  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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 I imagine I have made a mistake, but GHC shouldn't panic!

 {{{#!hs
 {-# language TypeInType, ScopedTypeVariables, MagicHash #-}

 import GHC.Exts (TYPE, Proxy#, proxy#)

 fold :: forall rep a (r :: TYPE rep).
           (r -> a -> Proxy# r -> r) -> (Proxy# r -> r) -> [a] -> r
 fold f k [] = k proxy#
 fold f k (x : xs) = fold f (f (k proxy#) x) xs
 }}}

 This gives me

 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.5.20171211 for x86_64-unknown-linux):
         splitFunTy
   ()
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:921:30 in ghc:Type
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14765>
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