ghci 8.0.2 panics

I know it is rather hard to reproduce but (I currently cannot check whether it still occurs in 8.0.2+):

` git clone https://github.com/jakubdaniel/expressions git clone https://github.com/jakubdaniel/expressions-z3 hg clone https://bitbucket.org/jd823592/z3-haskell wget https://github.com/Z3Prover/z3/releases/download/z3-4.4.1/z3-4.4.1-x64-ubuntu-14.04.zip unzip z3-4.4.1-x64-ubuntu-14.04.zip

cd expressions-z3
cabal sandbox init
cabal sandbox add-source ../expressions
cabal sandbox add-source ../z3-haskell
cabal install --extra-include-dirs=$PWD/../z3-4.4.1-x64-ubuntu-14.04/include --extra-lib-dirs=$PWD/../z3-4.4.1-x64-ubuntu-14.04/bin && cabal exec ghci

λ> -- forget to :set -XTypeInType
λ> :m Z3.Monad Data.Expression
λ> -- forget to :m Data.Expression.Z3
λ> evalZ3 $ astToString =<< toZ3 (forall [var "x" :: Var 'IntegralSort] (exists [var "y" :: Var 'IntegralSort] (var "x" .+. var "y" .=. cnst 0)) :: Lia 'BooleanSort)
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.2 for x86_64-unknown-linux):
        initTc: unsolved constraints
  WC {wc_insol =
        [W] toZ3_a4CL :: t_a4CK[tau:1] (CHoleCan: toZ3)
        [W] toZ3_a4Dx :: t_a4Dw[tau:1] (CHoleCan: toZ3)}

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