No coercion from from mandatory to optional field

Author: cristianoc

jscomp/ml/ctype.ml

@@ -3969,8 +3969,8 @@ let rec subtype_rec env trace t1 t2 cstrs =
           let label_decl_sub (acc1, acc2) ld2 =
             match Ext_list.find_first fields1 (fun ld1 -> ld1.ld_id.name = ld2.ld_id.name) with
             | Some ld1 ->
-              if field_is_optional ld1.ld_id repr1 && not (field_is_optional ld2.ld_id repr2) then
-                (* optional field can't be cast to non-optional one *)
+              if field_is_optional ld1.ld_id repr1 <> (field_is_optional ld2.ld_id repr2) then
+                (* optional field can't be modified *)
                 violation := true;
               let get_as (({txt}, payload) : Parsetree.attribute) =
                 if txt = "as" then Ast_payload.is_single_string payload

jscomp/test/RecordCoercion.res

@@ -6,14 +6,14 @@ type r3 = {a?: int}
 
 type r4 = {}
 
-let _ = (x: r1) => (x :> r2) // Convert a from mandatory to optional
+// let _ = (x: r1) => (x :> r2) // Convert a from mandatory to optional NOT ALLOWED
 // let _ = (x: r2) => (x :> r1) can't turn an optional field to a mandatory one
 let _ = (x: r2) => (x :> r3) // can omit field
-let _ = (x: r1) => (x :> r3) // omit field and convert from mandatory to optional
+// let _ = (x: r1) => (x :> r3) // omit field and convert from mandatory to optional NOT ALLOWED
 let _ = (x: r3) => (x :> r4) // omit everything
 
-type nested1 = {n: r1, extra: int}
-type nested2 = {n: r2}
+type nested1 = {n: r2, extra: int}
+type nested2 = {n: r3}
 let _ = (x: nested1) => (x :> nested2)
 
 module TestInlining = {