I get this error when compiling a program with associated type families:

$ ghc --make Queens_v2.hs 
[1 of 5] Compiling Domain           ( Domain.hs, Domain.o )
[2 of 5] Compiling Solver           ( Solver.hs, Solver.o )
[3 of 5] Compiling FD               ( FD.hs, FD.o )
[4 of 5] Compiling SearchTree       ( SearchTree.hs, SearchTree.o )
[5 of 5] Compiling Main             ( Queens_v2.hs, Queens_v2.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.1 for i386-apple-darwin):
	idInfo co{v a629} [tv]

Tom Schrijvers