Irrefutable pattern failed for pattern Data.Maybe.Just

[pganelin]

The following invalid program causes ghc panic

-----------------------
--State-Monad.hs
data Term =  Con Int
     | Div Term Term
     deriving (Show)

type M a =  State -> (a,State)
type State =Int

instance Monad Int -> (a,Int) where
    g >>= f = undefined
    return = undefined
-------------------------------

D:\>"C:\Free\Haskell Platform\2012.2.0.0\bin\ghc.exe" D:\State-Monad.hs
[1 of 1] Compiling Main             ( D:\State-Monad.hs, D:\State-Monad.o )
ghc.exe: panic! (the 'impossible' happened)
  (GHC version 7.4.1 for i386-unknown-mingw32):
        compiler\rename\RnSource.lhs:429:14-81: Irrefutable pattern failed for pattern Data.Maybe.Just (inst_tyvars,
                                                        _,
                                                        SrcLoc.L _ cls,
                                                        _)


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

[pcapriotti]

Thanks for the report. This is fixed in HEAD.