deriving instance Data (a->b) leads to 'impossible' error instead of "can't do" message

[claus]

$ cat Data.hs
{-# OPTIONS_GHC -fglasgow-exts #-}
import Data.Generics(Data)

deriving instance (Data a,Data b) => Data (a->b)

$ ghc --make Data.hs
[1 of 1] Compiling Main             ( Data.hs, Data.o )

Data.hs:4:0:ghc.exe: panic! (the 'impossible' happened)
  (GHC version 6.9.20080514 for i386-unknown-mingw32):
        newTyConEtadRhs ghc-prim:GHC.Prim.(->){(w) tc 3D}

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

[simonpj]

Yes, this should fail gracefully, thanks. Fixed by

Wed Jun 25 17:02:04 BST 2008  simonpj@microsoft.com
  * Fix Trac #2394: test for non-algebraic types in standalone deriving

Simon