HsOpTy is tied to Generic definitions which is not a very good design
feature, indeed a bug. However, the check is easy to move from
tcHsType back to mk_inst_info and everything will be fine. Also see
-bug #5.
+bug #5. [I don't think that this is the case anymore after SPJ's latest
+changes in that regard. Delete this comment? -=chak/7Jun2]
Generics.lhs