ghc panic
As ghc requested I am reporting this... All I can say right now is that I am tracking down a bug involving overlapping instances.
ghc: panic! (the 'impossible' happened)
(GHC version 7.4.1 for i386-unknown-linu
tcTyVarDetails
( t{tv acw0} [tv] :: ghc-prim:GHC.Prim.*{(w) tc 34d} )
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug