Re: [GHC] #14154: Some cocktail of features causes GHC panic

24 Aug 2017

      #14154: Some cocktail of features causes GHC panic
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by simonpj:

Old description:
...
{{{#!hs
{-# Language RankNTypes, DerivingStrategies, TypeApplications,
ScopedTypeVariables, GADTs, GeneralizedNewtypeDeriving, InstanceSigs,
PolyKinds #-}
import Data.Coerce
newtype Ran g h a = Ran (forall b. (a -> g b) -> h b)
newtype Swap p f g a where
  Swap :: p g f a -> Swap p f g a
  deriving newtype
    Show
class IxPointed m where
  ireturn :: a -> m i i a
instance IxPointed f => IxPointed (Swap f) where
  ireturn :: forall a i. a -> Swap f i i a
  ireturn = coerce (ireturn @f @a @i)
instance IxPointed Ran where
  ireturn :: a -> Ran i i a
  ireturn a = Ran (\k -> k a)
xs =
  case ireturn @(Swap Ran) 'a' of
    Swap (Ran f) -> f print
}}}
{{{
$ ghci -ignore-dot-ghci /tmp/bug.hs
GHCi, version 8.3.20170605: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( /tmp/bug.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.3.20170605 for x86_64-unknown-linux):
        piResultTy
  k0_a1Ki[tau:2]
  b0_a1Kt[tau:2]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at
compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:949:35 in ghc:Type
Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
...
}}}
New description:

 {{{#!hs
 {-# Language RankNTypes, DerivingStrategies, TypeApplications,
     ScopedTypeVariables, GADTs, PolyKinds #-}

 module T14154 where

 newtype Ran g h a
   = Ran (forall b. (a -> g b) -> h b)

 newtype Swap p f g a where
   Swap :: p g f a -> Swap p f g a

 ireturn :: forall m i a. a -> m i i a
 ireturn = undefined

 xs = case ireturn @(Swap Ran) 'a' of
        Swap (Ran f) -> f print
 }}}

 {{{
 $ ghci -ignore-dot-ghci /tmp/bug.hs
 GHCi, version 8.3.20170605: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/bug.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.3.20170605 for x86_64-unknown-linux):
         piResultTy
   k0_a1Ki[tau:2]
   b0_a1Kt[tau:2]
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:949:35 in ghc:Type

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

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14154#comment:3
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