I found the following results from strictness analyis:
  f x = error (fst x)      -- Strictness U(SA)b
  g x = error ('y':fst x)  -- Strictness Tb

Surely 'g' is no less strict on 'x' than 'f' is!  The fix turned out
be to very nice and simple.  See Note [Bottom demands] in DmdAnal.