Jackson 2.16 fails attempting to obtain `ObjectWriter` for an `Enum` which some value returns null from `toString()`

[YutaHiguchi-bsn]

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Describe the bug

After upgrading to 2.16.1, I cannot obtain ObjectWriter for enum classes, which some of the value returns null from toString().

This used to work in 2.15.3

Version Information

2.16.0, 2.16.1

Reproduction

Following is the minimum JUnit 5 reproducer.

This works fine on 2.15.3 but exceptionally fails at assertDoesNotThrow(..) on 2.16.0 or 2.16.1.

    enum NullToStringEnum {
        ALPHA("A"),
        BETA("B"),
        UNDEFINED(null);

        private final String s;

        NullToStringEnum(String s) {
            this.s = s;
        }

        @Override
        public String toString() {
            return s;
        }
    }

    @Test
    void nullToStringEnum() {
        ObjectMapper mapper = new ObjectMapper();
        assertDoesNotThrow(() -> mapper.writerFor(NullToStringEnum.class));
        assertEquals("\"ALPHA\"", w.writeValueAsString(NullToStringEnum.ALPHA));
        assertEquals("\"UNDEFINED\"", w.writeValueAsString(NullToStringEnum.UNDEFINED));
    }

backtrace looks like:

Caused by: java.lang.IllegalStateException: Null String illegal for SerializedString
	at com.fasterxml.jackson.core.io.SerializedString.<init>(SerializedString.java:53)
	at com.fasterxml.jackson.databind.cfg.MapperConfig.compileString(MapperConfig.java:245)
	at com.fasterxml.jackson.databind.util.EnumValues.constructFromToString(EnumValues.java:136)
	at com.fasterxml.jackson.databind.ser.std.EnumSerializer.construct(EnumSerializer.java:125)
	at com.fasterxml.jackson.databind.ser.BasicSerializerFactory.buildEnumSerializer(BasicSerializerFactory.java:1218)
	at com.fasterxml.jackson.databind.ser.BasicSerializerFactory.findSerializerByPrimaryType(BasicSerializerFactory.java:428)
	at com.fasterxml.jackson.databind.ser.BeanSerializerFactory._createSerializer2(BeanSerializerFactory.java:235)
	at com.fasterxml.jackson.databind.ser.BeanSerializerFactory.createSerializer(BeanSerializerFactory.java:174)
	at com.fasterxml.jackson.databind.SerializerProvider._createUntypedSerializer(SerializerProvider.java:1525)
	at com.fasterxml.jackson.databind.SerializerProvider._createAndCacheUntypedSerializer(SerializerProvider.java:1493)
	at com.fasterxml.jackson.databind.SerializerProvider.findValueSerializer(SerializerProvider.java:619)
	at com.fasterxml.jackson.databind.SerializerProvider.findTypedValueSerializer(SerializerProvider.java:901)
	at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.forRootType(ObjectWriter.java:1535)
	at com.fasterxml.jackson.databind.ObjectWriter.<init>(ObjectWriter.java:116)
	at com.fasterxml.jackson.databind.ObjectMapper._newWriter(ObjectMapper.java:838)
	at com.fasterxml.jackson.databind.ObjectMapper.writerFor(ObjectMapper.java:4135)
	at org.example.MainTest.lambda$nullToStringEnum$0(MainTest.java:31)
	at org.junit.jupiter.api.AssertDoesNotThrow.assertDoesNotThrow(AssertDoesNotThrow.java:71)

Expected behavior

Be able to serialize Enums even though it's #toString() returns null

Additional context

Returning null from toString() is probably bad practice, but such Enums are out there in the wild.

From the 2.16.1 backtrace, it seems to be related to the change #4039

Building EnumValues valuesByToString regardless of the SerializationFeature.WRITE_ENUMS_USING_TO_STRING config might be the issue?

[cowtowncoder]

I agree, failure should not occur especially when there is no need to call Enum.toString() in this case.
This despite it being illegal (as far I understand) for toString() to return null.