Irrefutable pattern failed for pattern Data.Maybe.Just
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