fix: merge the right fixity information

typing/btype.ml

@@ -167,8 +167,8 @@ let rec row_more row =
   | {desc=Tvariant row'} -> row_more row'
   | ty -> ty
 
-let merge_fixed_explanation row1 row2 =
-  match row1.row_fixed, row2.row_fixed with
+let merge_fixed_explanation fixed1 fixed2 =
+  match fixed1, fixed2 with
   | Some Univar _ as x, _ | _, (Some Univar _ as x) -> x
   | Some Fixed_private as x, _ | _, (Some Fixed_private as x) -> x
   | Some Reified _ as x, _ | _, (Some Reified _ as x) -> x
@@ -447,7 +447,7 @@ let copy_row f fixed row keep more =
         | Rpresent(Some ty) -> Rpresent(Some(f ty))
         | Reither(c, tl, m, e) ->
             let e = if keep then e else ref None in
-            let m = if row.row_fixed <> None then fixed else m in
+            let m = if is_fixed row then fixed else m in
             let tl = List.map f tl in
             Reither(c, tl, m, e)
         | _ -> fi)

typing/btype.mli

@@ -72,13 +72,18 @@ val row_more: row_desc -> type_expr
 
 val is_fixed: row_desc -> bool
 (* Return whether the row is directly marked as fixed or not *)
+
 val row_fixed: row_desc -> bool
 (* Return whether the row should be treated as fixed or not.
    In particular, [is_fixed row] implies [row_fixed row].
 *)
+
 val fixed_explanation: row_desc -> fixed_explanation option
 (* Return the potential explanation for the fixed row *)
-val merge_fixed_explanation: row_desc -> row_desc -> fixed_explanation option
+
+val merge_fixed_explanation:
+  fixed_explanation option -> fixed_explanation option
+  -> fixed_explanation option
 (* Merge two explanations for a fixed row *)
 
 val static_row: row_desc -> bool

typing/ctype.ml

@@ -2815,7 +2815,7 @@ and unify_row env row1 row2 =
     | None, Some _ -> rm2
     | None, None -> newty2 (min rm1.level rm2.level) (Tvar None)
   in
-  let fixed = merge_fixed_explanation row1 row2
+  let fixed = merge_fixed_explanation fixed1 fixed2
   and closed = row1.row_closed || row2.row_closed in
   let keep switch =
     List.for_all