Bad rule generated in pathological cases
George reports: I am also seeing this on ghc 7.10.1, not sure if this is another instance of this bug (#9160 (closed)):
cabal install bitmap [ 3 of 10] Compiling Data.Bitmap.IO ( Data/Bitmap/IO.hs, dist/build/Data/Bitmap/IO.o ) Data/Bitmap/IO.hs:1248:1: Warning: SPECIALISE pragma for non-overloaded function ‘myPlusPtr’ Data/Bitmap/IO.hs:1249:1: Warning: SPECIALISE pragma for non-overloaded function ‘myPlusPtr’ ghc: panic! (the 'impossible' happened) (GHC version 7.10.1 for x86_64-apple-darwin): Template variable unbound in rewrite rule $fPixelComponentFloat3_X2Rc [$fPixelComponentFloat3_X2Rc] [$fPixelComponentFloat3_X2Rc]  
But it's not #9160 (closed) at all. So I'm making a new ticket.