Disabling `FAIL_ON_INVALID_SUBTYPE` breaks polymorphic deserialization of Enums

[holgerknoche]

Apparently, disabling the FAIL_ON_INVALID_SUBTYPE feature has some (at least for me) unexpected side-effects on the deserialization of enums. I originally found this issue in jackson-databind 2.10.0, but it still exists in 2.11.0.
I have an enum that implements an interface:

@JsonTypeInfo(use = Id.NAME)
public interface CustomEnum {}

@JsonTypeName("Test")
public enum TestEnum implements CustomEnum {
    VALUE;
}

Then, this test case works fine:

ObjectMapper mapper = new ObjectMapper();
mapper.registerSubtypes(TestEnum.class);
	
mapper.enable(DeserializationFeature.FAIL_ON_INVALID_SUBTYPE);
	
CustomEnum testValue = TestEnum.VALUE;
String json = mapper.writeValueAsString(testValue);
	
CustomEnum deserializedValue = mapper.readValue(json, CustomEnum.class);
assertEquals(testValue, deserializedValue);

However, when I disable the feature, the test case fails because deserializedValue is suddenly null. The reason seems to be that a NullyfingDeserializer is returned from TypeDeserializerBase._findDefaultImplDeserializer(...) instead of null. Therefore, the AsPropertyDeserializer no longer tries to read the value itself.

[singleton11]

public class Test {

    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME)
    interface CustomEnum {}

    @JsonTypeName("Test")
    enum TestEnum implements CustomEnum {
        VALUE;
    }

    @org.junit.Test
    public void test() throws JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        mapper.registerSubtypes(TestEnum.class);

//        mapper.enable(DeserializationFeature.FAIL_ON_INVALID_SUBTYPE);

        CustomEnum testValue = TestEnum.VALUE;
        String json = mapper.writeValueAsString(testValue);

        CustomEnum deserializedValue = mapper.readValue(json, CustomEnum.class);
        assertEquals(testValue, deserializedValue);
    }
}

This test works fine in 2.11 branch, what I do wrong?

UPD: Sorry, I see, I just had to explicitly disable this feature

[cowtowncoder]

Interesting. I can reproduce this. Workaround that trips handling appears to be fix for #148 so can not just remove use of NullifyingDeserializer.

[cowtowncoder]

Fundamentally the problem is that Enums are serialized as Strings -- scalar value -- and not as JSON Objects, so it is not possible use inclusion of As.PROPERTY. Instead, serialization occurs using As.WRAPPER_ARRAY.
But on deserialization this can not be determined. I am not sure why it appears to work with different settings however.

A solution here really is to use different inclusion:

    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.WRAPPER_ARRAY)

and things will work.

It is odd that without disabling FAIL_ON_INVALID_SUBTYPE things appear to work ok, so I'll see if there is a way to handle this case as expected. But in the mean time specifying more general inclusion type (WRAPPER_ARRAY is guaranteed to work all shapes, ditto for WRAPPER_OBJECT).

[cowtowncoder]

I was able to fix this, but since there is some risk in fix will only add in for 2.12.0 (probably in 2.12.0-rc2), not for earlier minor versions. I suspect risk simply because this affects edge cases and testing is probably not extensive for such cases.