Started getting:

ghc-6.2.1.20040313: panic! (the `impossible' happened, GHC version 6.2.1.20040313): mkTyVar a1 {- tv a75q -}

Please report it as a compiler bug to glasgow-haskell-bugs@haskell.org, or http://sourceforge.net/projects/ghc/.

today... definitely a but to do with class inference, as it started appearing aftyer changing some instance constraints and fundeps on classes.

Can't really be a lot more specific as code is big. Let me know if you want me to produce a small code extract which triggers it.

Could you send us enough code to reproduce the bug? The smaller the better, but don't worry if you can't reduce it. Cheers, Simon