GHC panic

[guest]

data RunLength = Single Char | Multiple (Char, Int)  deriving(show)
encode'        :: String -> [RunLength]
encode'     [] = []
encode'  (x:xs) = if len == 1 then Single x : encode' xs else
                     Multiple (x, len) : encode' rest
                where (first, rest) = span (==x) xs
                      len = length first


[1 of 1] Compiling Main             ( 99_1.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 7.4.2 for x86_64-apple-darwin):
	nameModule show{tv abq4}

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

I notice I mistyped Show as show, but I think it's better to report it since the compiler doesnt catch the error

[simonpj]

Thanks; this is a duplicate of #7183, now fixed.