[GHC] #16135: Panic with ExistentialQuantification and ApplicativeDo

[GHC]

#16135: Panic with ExistentialQuantification and ApplicativeDo
-------------------------------------+-------------------------------------
        Reporter:  Ashley Yakeley    |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------
Description changed by Ashley Yakeley:

Old description:

> {{{
> {-# 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
> }}}

New description:

 {{{#!hs
 {-# 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#comment:1>
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