[GHC] #16135: Panic with ExistentialQuantification and ApplicativeDo

[GHC]

#16135: Panic with ExistentialQuantification and ApplicativeDo
-------------------------------------+-------------------------------------
           Reporter:  Ashley         |             Owner:  (none)
  Yakeley                            |
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           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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 {{{
 {-# LANGUAGE ExistentialQuantification, ApplicativeDo #-}

 module Bug where

 data T f = forall a. MkT (f a)

 runf :: forall f. Functor f => IO (T f)
 runf = do
     return ()
     MkT fa <- runf
     return $ MkT fa
 }}}

 {{{

 Bug.hs:11:18: error:ghc: panic! (the 'impossible' happened)
   (GHC version 8.6.3 for x86_64-unknown-linux):
         No skolem info:
   [a_a1nb[ssk:2]]
   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

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16135>
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