What is tested here:

Due to a change in GHC 7.6.1 we had a bug that superclass contraints were included in the instances list. Edward K. repported it here:

http://www.haskell.org/pipermail/haskell-cafe/2012-September/103600.html

And here is the corresponding theard on glasgow-haskell-users:

http://www.haskell.org/pipermail/glasgow-haskell-users/2012-September/022914.html

It has been fixed in:

 6ccf78e15a525282fef61bc4f58a279aa9c21771
 Fix spurious superclass constraints bug.