Yet another correction to earlier commit..
*now* those rank-2 pattern matches should work
MERGE TO STABLE
-- Check the fields
tc_fields field_tys rpats `thenTc` \ (rpats', lie_req3, tvs, ids, lie_avail2) ->
- returnTc (RecPat data_con con_res_ty ex_tvs ex_dicts rpats',
+ returnTc (co_fn <$> RecPat data_con con_res_ty ex_tvs ex_dicts rpats',
lie_req1 `plusLIE` lie_req2 `plusLIE` lie_req3,
listToBag ex_tvs `unionBags` tvs,
ids,