The issue here was: what import declaration brings into
scope the 'op here

   import qualified Foo( op )
   import Bar( C(op) )
   instance C Int where
     op = ...

Well, the import of Bar, obviously.  But what if the
import Bar had been
   import Bar( C )
Then the instance is still supposed to work, getting
op from the Foo.op imported from Foo.  (I'm assuming its
the same op, of course.)