BUG properties of type Object do not have any type information when generating open api 3.1.0

[Lorenzo-Bracci]

Problem

When generating OpenAPI 3.1.0 spec, properties with the java type Object do not have any type information in the generated open api but are instead empty. As an example assume the following java model:

private static class PojoUsingObjectField {
        private Object myField;

        public Object getMyField() {
            return myField;
        }

        public void setMyField(Object myField) {
            this.myField = myField;
        }
}

When generating open api for 3.0.1 the generated model would be:

PojoUsingObjectField:
  type: object
  properties:
    myField:
      type: object

But when generating open api 3.1.0 the generated model is:

PojoUsingObjectField:
  type: object
  properties:
    myField: {}

Expected behaviour

Same as in open api 3.0.1, properties have a type field with value object.

Reproducer

The issues is reproduced in the following test (note that the test is successful if the model resolvers generate OAS3.0.1 instead):

    @Test(description = "Shows that type information in properties using a raw Object is lost when using oas 3.1.0")
    public void testModelUsingObjectTypedPropertyLosesTypeInformationForOas31() {

        String expectedYaml = "PojoUsingObjectField:\n" +
                "  type: object\n" +
                "  properties:\n" +
                "    myField:\n" +
                "      type: object";

        Map<String, io.swagger.v3.oas.models.media.Schema> stringSchemaMap = ModelConverters.getInstance(true).readAll(PojoUsingObjectField.class);
        // fails as the type: object will not be added for oas 3.1.0, instead we will have "myField: {}"
        SerializationMatchers.assertEqualsToYaml31(stringSchemaMap, expectedYaml);
    }

    private static class PojoUsingObjectField {
        private Object myField;

        public Object getMyField() {
            return myField;
        }

        public void setMyField(Object myField) {
            this.myField = myField;
        }
    }

Investigation

Looking at the ModelResolver code, it seems that the issue is the following: the model for a property with java type Object is generated using PrimitiveType.OBJECT#createProperty which creates a io.swagger.v3.oas.models.media.Schema with type set to object (i.e using oas 3.0.1 representation). Nevertheless in ModelResolver#717 we clone this representation before further processing and because cloning is done by serializing and then deserializing using the jackson settings for oas 3.1.0 (which expects the property types to be used instead of type) this information is lost and the cloned schema has neither type nor types set