[GHC] #15598: RebindableSyntax with RankNTypes and type class method call yields panic.

3 Sep 2018

      #15598: RebindableSyntax with RankNTypes and type class method call yields panic.
-------------------------------------+-------------------------------------
           Reporter:  romanb         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Linux
  RebindableSyntax, RankNTypes       |
       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 in a file `ghc-panic.hs`

 {{{

 {-# LANGUAGE
     GADTSyntax
   , RankNTypes
   , RebindableSyntax
 #-}

 import Prelude hiding ((>>=))

 data InfDo where
     InfDo :: String -> (forall a. a -> InfDo) -> InfDo

 prog :: InfDo
 prog = do
     _ <- show (42 :: Int)
     prog
   where
     (>>=) = InfDo

 main :: IO ()
 main = let x = prog in x `seq` return ()
 }}}

 when loaded into GHCi yields

 {{{
 λ> main
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.4.3 for x86_64-unknown-linux):
         nameModule
   system $dShow_abfY
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/basicTypes/Name.hs:241:3 in ghc:Name
 }}}

 and separate compilation yields

 {{{
 $ ghc ghc-panic.hs
 [1 of 1] Compiling Main             ( ghc-panic.hs, ghc-panic.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.4.3 for x86_64-unknown-linux):
         StgCmmEnv: variable not found
   $dShow_a1qI
   local binds for:
   $tcInfDo
   $trModule
   $tcInfDo1_r1th
   $tcInfDo2_r1tH
   $trModule1_r1tI
   $trModule2_r1tJ
   $trModule3_r1tK
   $trModule4_r1tL
   sat_s1E4
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/codeGen/StgCmmEnv.hs:149:9 in
 ghc:StgCmmEnv
 }}}

 The problem disappears when either the rank-2 type is removed from `InfDo`
 or when the call to `show` is replaced by a static string.

 Besides 8.4.3, also reproduced with 8.6.0.20180714

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15598
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