Fix ord variant wildcard_case to_int type pre-OCaml 4.11 (PR ocaml-ppx#260)

Author: sim642

src_plugins/ord/ppx_deriving_ord.cppo.ml

@@ -37,8 +37,15 @@ let reduce_compare l =
 
 let wildcard_case ~typ int_cases =
   let loc = !Ast_helper.default_loc in
+  let to_int_fun = [%expr fun (x: [%t typ]) -> [%e Exp.match_ [%expr x] int_cases]] in
+  (* Need explicit polymorphic type for pre-OCaml 4.11 compatibility,
+     but just using Ppx_deriving.strong_type_of_type doesn't work,
+     so adding explicit polymorphism using (type a) arguments instead.
+     See: https://github.com/ocaml-ppx/ppx_deriving/pull/260#issuecomment-1072995847. *)
+  let free_vars = Ppx_deriving.free_vars_in_core_type typ in
+  let poly_to_int_fun = List.fold_right Exp.newtype free_vars to_int_fun in
   Exp.case [%pat? _] [%expr
-    let to_int: [%t typ] -> Ppx_deriving_runtime.int = [%e Exp.function_ int_cases] in
+    let to_int = [%e poly_to_int_fun] in
     Ppx_deriving_runtime.compare (to_int lhs) (to_int rhs)]
 
 let pattn side typs =