[Haskell-cafe] Undecidable instances with functional dependencies

I have the following class and instance class Register a r | a -> r where instance (Register a ra, Register b rb) => Register (a,b) (ra,rb) where and GHC refuses the instance because of violated Coverage Condition. I have more instances like instance Register Int8 (Reg Int8) where instance Register Word8 (Reg Word8) where and for the set of instances I plan, the instance resolution will always terminate. I remember that the term 'undecidable instance' is not fixed and may be relaxed if a more liberal condition can be found. Is there a place, say a Wiki page, where we can collect examples where we think that the current check of GHC is too restrictive?