I have no insight into what is hard and what is easy to implement here, but might it be feasible to break up the error into parts, the first just as you wrote? Along the lines of:

1. Error: This expression has type ('x -> Result_set.t) list but an expression was expected of type Result_set.t list

2. where 'x = (locl_ty * Tast.pos * ('ex, 'fb, 'en) Aast.expr_) list

The solution suggested by the parent of your post sounds desirable also, but perhaps the currying semantics of Ocaml makes that difficult or poorly-defined?