subtype_row: filter out absent fields when the other row is closed

testsuite/tests/typing-polyvariants-bugs/pr7824.ml

@@ -56,13 +56,6 @@ let f x =
 ;;
 [%%expect{|
 type _ t = T : 'a -> 'a t
-Line _, characters 11-37:
-    | T _ -> (x :> [`A | `C] Element.t)
-             ^^^^^^^^^^^^^^^^^^^^^^^^^^
-Error: Type [ `A ] Element.t is not a subtype of [ `A | `C ] Element.t
-       The first variant type does not allow tag(s) `C
-|}, Principal{|
-type _ t = T : 'a -> 'a t
 val f : [ `A ] Element.t -> [ `A | `C ] Element.t = <fun>
 |}];;
 
@@ -72,11 +65,7 @@ let f () =
   (x :> [ `A | `C ] Element.t)
 ;;
 [%%expect{|
-Line _, characters 2-30:
-    (x :> [ `A | `C ] Element.t)
-    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-Error: Type [ `A ] Element.t is not a subtype of [ `A | `C ] Element.t
-       The first variant type does not allow tag(s) `C
+val f : unit -> [ `A | `C ] Element.t = <fun>
 |}];;
 
 let f () =

typing/ctype.ml

@@ -4121,6 +4121,8 @@ and subtype_row env trace row1 row2 cstrs =
   let row1 = row_repr row1 and row2 = row_repr row2 in
   let r1, r2, pairs =
     merge_row_fields row1.row_fields row2.row_fields in
+  let r1 = if row2.row_closed then filter_row_fields false r1 else r1 in
+  let r2 = if row1.row_closed then filter_row_fields false r2 else r2 in
   let more1 = repr row1.row_more
   and more2 = repr row2.row_more in
   match more1.desc, more2.desc with