There it goes, J.A. type CTPar = ([Double],Int,Int) us :: (?ctPar :: CTPar) => [Double] us = let (us',_,_) = ?ctPar in us' ghc-5.04: panic! (the `impossible' happened, GHC version 5.04): tcSplitTyConApp {?ctPar{-r2kd-} :: ([GHCziFloat.Double{-3u-}], GHCziBase.Int{-3J-}, GHCziBase.Int{-3J-})} Please report it as a compiler bug to glasgow-haskell-bugs@haskell.org, or http://sourceforge.net/projects/ghc/.