Panic (core lint failure) with type synonym in GHC 6.10.1

[BenMoseley]

This causes a panic:

{-# LANGUAGE TypeFamilies, GADTs #-}
module GHCBug (
  PVR(..),
  Core(..),
  analyseCore
)
where

data Core a where Ctr :: Core Double
data PVR a = PVR a deriving (Eq, Show)


class Sub a where
    type AssocSyn a :: * -> *

instance Sub Double where
    type AssocSyn Double = PVR


analyseCore :: Core a -> ((AssocSyn a) a)
analyseCore Ctr = pvr
  where
    -- GHC panics if we use the below as the type sig for 'pvr'
    pvr :: PVR ~ AssocSyn a => (AssocSyn a) a
    -- pvr :: (AssocSyn a) a
    pvr = undefined

main :: IO ()
main = print "ok"

The basic compiler panic is:

c:/ws/main/depot/QA/EDG/EDG_priv/FPF_Dev.br/src $ ghc -main-is GHCBug ~/GHCBug.hs
ghc.exe: panic! (the 'impossible' happened)
  (GHC version 6.10.1 for i386-unknown-mingw32):
	initC: srt_lbl

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

The core lint failure part is:

*** Checking old interface for main:GHCBug:
*** Parser:
*** Renamer/typechecker:
*** Desugar:
    Result size = 199
*** Core Lint Errors: in result of Desugar ***
{-# LINE 21 "F:\ME\GHCBug.hs #-}:
    [RHS of pvr_awa :: GHCBug.AssocSyn
                         GHC.Types.Double GHC.Types.Double]
    pvr_afN is out of scope
*** Offending Program ***

[BenMoseley]

Full Output

[chak]

Thanks for the bug report. In your code example, you could write

  ...
  where
     pvr :: PVR a
     pvr = undefined

That signature would be equivalent to the one that causes the panic.

I am just suggesting this as a possible work around until GHC has been fixed, but maybe it is only applicable to the stripped down example of your bug report.

[BenMoseley]

Thanks for the response. I've managed to work around it without too much trouble, so it's not directly urgent for us at the moment.

[simonpj]

Happily this seems to be fixed by the same fix as #2799, so I'll close this one. I won't add a test case, since I added one for #2799.

Simon