+We have to be very, very careful when generating superclasses, lest we
+accidentally build a loop. Here's an example:
+
+ class S a
+
+ class S a => C a where { opc :: a -> a }
+ class S b => D b where { opd :: b -> b }
+
+ instance C Int where
+ opc = opd
+
+ instance D Int where
+ opd = opc
+
+From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
+Simplifying, we may well get:
+ $dfCInt = :C ds1 (opd dd)
+ dd = $dfDInt
+ ds1 = $p1 dd
+Notice that we spot that we can extract ds1 from dd.
+
+Alas! Alack! We can do the same for (instance D Int):
+
+ $dfDInt = :D ds2 (opc dc)
+ dc = $dfCInt
+ ds2 = $p1 dc
+
+And now we've defined the superclass in terms of itself.
+
+
+Solution: treat the superclass context separately, and simplify it
+all the way down to nothing on its own. Don't toss any 'free' parts
+out to be simplified together with other bits of context.
+Hence the tcSimplifyTop below.
+
+At a more basic level, don't include this_dict in the context wrt
+which we simplify sc_dicts, else sc_dicts get bound by just selecting
+from this_dict!!
+
+\begin{code}
+tcSuperClasses inst_tyvars' dfun_arg_dicts sc_dicts
+ = tcAddErrCtxt superClassCtxt $
+ tcSimplifyCheck doc inst_tyvars'
+ dfun_arg_dicts
+ (mkLIE sc_dicts) `thenTc` \ (sc_lie, sc_binds1) ->
+
+ -- It's possible that the superclass stuff might have done unification
+ checkSigTyVars inst_tyvars' `thenTc` \ zonked_inst_tyvars ->
+
+ -- We must simplify this all the way down
+ -- lest we build superclass loops
+ tcSimplifyTop sc_lie `thenTc` \ sc_binds2 ->
+
+ returnTc (zonked_inst_tyvars, sc_binds1, sc_binds2)
+
+ where
+ doc = ptext SLIT("instance declaration superclass context")
+\end{code}
+
+\begin{code}
+mkMethodBinds clas inst_tys' op_items monobinds
+ = -- Check that all the method bindings come from this class
+ mapTc (addErrTc . badMethodErr clas) bad_bndrs `thenNF_Tc_`
+
+ -- Make the method bindings
+ mapAndUnzipTc mk_method_bind op_items
+
+ where
+ mk_method_bind op_item = mkMethodBind InstanceDeclOrigin clas
+ inst_tys' monobinds op_item
+
+ -- Find any definitions in monobinds that aren't from the class
+ sel_names = [idName sel_id | (sel_id, _) <- op_items]
+ bad_bndrs = collectMonoBinders monobinds `minusList` sel_names
+\end{code}
+
+