- ; gbl_tvs <- tcGetGlobalTyVars
- ; let preds = fdPredsOfInsts irreds2 -- irreds2 is zonked
- qtvs = grow preds tau_tvs2 `minusVarSet` oclose preds gbl_tvs
+ ; gbl_tvs2 <- zonkTcTyVarsAndFV (varSetElems gbl_tvs1)
+ -- Note that we start from gbl_tvs1
+ -- We use tcGetGlobalTyVars, then oclose wrt preds2, because
+ -- we've already put some of the original preds1 into frees
+ -- E.g. wanteds = C a b (where a->b)
+ -- gbl_tvs = {a}
+ -- tau_tvs = {b}
+ -- Then b is fixed by gbl_tvs, so (C a b) will be in free, and
+ -- irreds2 will be empty. But we don't want to generalise over b!
+ ; let preds2 = fdPredsOfInsts irreds2 -- irreds2 is zonked
+ qtvs = grow preds2 tau_tvs2 `minusVarSet` oclose preds2 gbl_tvs2