crash caused by generic visitor (?)

[Ranjit Jhala]

Hi all,

I'm trying to extract the set of identifiers that are read in given 
source file.  To this end, I wrote the following code (full source at end.)

------------------------------------------------------------------------
main 
  = do fname   <- (!! 0) `fmap` getArgs
       tcm     <- loadTypecheckedSource fname
       putStrLn $ showPpr tcm           -- this works fine
       putStrLn $ showPpr $ allIds tcm  -- this causes the crash
       return ()
  
allIds ::  Data a => a -> [Id]
allIds = listify (\x -> case (x :: Id) of _ -> True)
------------------------------------------------------------------------

and where:

	loadTypecheckedSource ::  FilePath -> IO TypecheckedSource

unfortunately, when I compile and run it, I get the dreaded:

	Bug: Bug: panic! (the 'impossible' happened)
  	(GHC version 7.0.3 for i386-unknown-linux):
		placeHolderNames

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

Turns out that the problem is when the file contains a type annotation.
That is, 

	./Bug Test00.hs

crashes, when Test00.hs is:

	module Test where
	
	x :: Int
	x = 0

but does not crash when the file is:

	module Test where
	
	x = 0

Can anyone tell me why listify chokes in the latter case? (And how one might 
get around the problem?) I include the full source below (compiled with: ghc --make Bug, using ghc 7.0.3)

Thanks!

Ranjit.

------------------------------------------------------------------------------------------------------
import GHC 
import Outputable
import DynFlags (defaultDynFlags)
import GHC.Paths (libdir)

import System.Environment (getArgs)
import Control.Monad
import qualified Data.List as L
import Data.Data
import Data.Generics.Schemes (listify)

main 
  = do fname   <- (!! 0) `fmap` getArgs
       tcm     <- loadTypecheckedSource fname
       putStrLn $ showPpr tcm           -- this works fine
       putStrLn $ showPpr $ allIds tcm  -- this causes the crash
       return ()
  
allIds ::  Data a => a -> [Id]
allIds = listify (\x -> case (x :: Id) of _ -> True)

loadTypecheckedSource ::  FilePath -> IO TypecheckedSource
loadTypecheckedSource fname 
  = defaultErrorHandler defaultDynFlags $ 
      runGhc (Just libdir) $ do
        df  <- getSessionDynFlags
        setSessionDynFlags df
        tgt <- guessTarget fname Nothing
        setTargets [tgt]
        load LoadAllTargets 
        res <- load LoadAllTargets
        if failed res
          then pprPanic "Load Failed!!" (text "AAARGH!")
          else tm_typechecked_source `fmap` getTypecheckedModule fname 

findModSummary ::  GhcMonad m => FilePath -> m ModSummary
findModSummary fname 
  = do msums <- depanal [] False
       case L.find ((fname ==) . ms_hspp_file) msums of 
         Just msum -> return msum
         Nothing   -> pprPanic "ModuleName Lookup Failed!!" (text "AARGHC!")

getTypecheckedModule :: GhcMonad m => FilePath -> m TypecheckedModule
getTypecheckedModule = findModSummary >=> parseModule >=> typecheckModule