Fix incorrect sort in type_argument (#3972)

Correct sort in coercion case of [type_argument]

Author: ccasin

testsuite/tests/typing-layouts/omitted_arguments.ml

@@ -72,3 +72,17 @@ let evaluate =
 let _ =
   Printf.printf "\"real\" code (3.14): %.2f\n"
     (box_float (evaluate [|2; 3; 4|] ~size:1))
+
+(* Omitting optional arg, where let binding is needed for function. *)
+module M = struct
+  (* Putting this in a module defeats an optimization in simplif that removes an
+     intermediate let binding where the bug occurs (it reduces [let x = y in
+     ...], but only if [y] is precisely a variable, which a projection from a
+     module is not). *)
+  let to_string ?(explicit_plus = false) x =
+    if explicit_plus then "blah" else string_of_float (box_float x)
+end
+
+let pi_to_string ~value_to_string = value_to_string #3.14
+let pi = pi_to_string ~value_to_string:M.to_string
+let () = Printf.printf "Omitting optional arg (3.14): %s\n" pi

testsuite/tests/typing-layouts/omitted_arguments.reference

@@ -2,3 +2,4 @@ Omitting named arg (6.29): 6.29
 Omitting named arg (6.27): 6.27
 Omitting named arg (9.42): 9.42
 "real" code (3.14): 3.14
+Omitting optional arg (3.14): 3.14

typing/typecore.ml

@@ -8146,10 +8146,16 @@ and type_argument ?explanation ?recarg ~overwrite env (mode : expected_mode) sar
           (Warnings.Non_principal_labels "eliminated omittable argument");
       (* let-expand to have side effects *)
       let let_pat, let_var = var_pair ~mode:exp_mode "arg" texp.exp_type in
+      let let_pat_sort =
+        (* The sort of the let-bound variable, which here is always a function
+           (observe it is passed to [func], which builds an application of
+           it). *)
+        Jkind.Sort.value
+      in
       re { texp with exp_type = ty_fun;
              exp_desc =
                Texp_let (Nonrecursive,
-                         [{vb_pat=let_pat; vb_expr=texp; vb_sort=arg_sort;
+                         [{vb_pat=let_pat; vb_expr=texp; vb_sort=let_pat_sort;
                            vb_attributes=[]; vb_loc=Location.none;
                            vb_rec_kind = Dynamic;
                           }],