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.