Enum aliases from an appended AnnotationIntrospector are discarded: JacksonAnnotationIntrospector.findEnumAliases() overwrites every slot #6339

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Describe the bug

A module that adds an AnnotationIntrospector with appendAnnotationIntrospector() (so it becomes the secondary of an AnnotationIntrospectorPair) cannot contribute Enum aliases. Its aliases are ignored and the alias is rejected:

tools.jackson.databind.exc.InvalidFormatException: Cannot deserialize value of type `...$Color` from String "rouge": not one of the values accepted for Enum class: [RED, GREEN]

The same introspector works when added with insertAnnotationIntrospector().

Cause: AnnotationIntrospectorPair.findEnumAliases() calls the secondary first and then the primary, "to give _primary higher precedence" (AnnotationIntrospectorPair.java:570-571 on 3.x, :599-600 on 2.x). That only works if the primary leaves slots without @JsonAlias unchanged. JacksonAnnotationIntrospector.findEnumAliases() instead assigns every slot:

aliasList[i] = enumToAliasMap.getOrDefault(enumValue.name(), new String[]{});

(JacksonAnnotationIntrospector.java:231 on 3.x, :289 on 2.x). This replaces whatever the secondary introspector set with an empty array. The older Class-based variant (2.x, JacksonAnnotationIntrospector.java:252-270) and findEnumValues() (:207-213) only assign when an annotation is present. The AnnotatedClass variant came in with #3832 (2.15), so this has probably been there since 2.15.

Version Information

3.x (3.3.0-SNAPSHOT, 4f153f0), 3.2.4, 2.x (2.23.0-SNAPSHOT, 425299b), 2.22.4.

Reproduction

enum Color { RED, GREEN }

static class AliasIntrospector extends NopAnnotationIntrospector {
    @Override
    public Version version() { return Version.unknownVersion(); }

    @Override
    public void findEnumAliases(MapperConfig<?> config, AnnotatedClass ac,
            Enum<?>[] enumValues, String[][] aliases) {
        aliases[0] = new String[] { "rouge" };
    }
}

static class AliasModule extends SimpleModule {
    @Override
    public void setupModule(SetupContext context) {
        super.setupModule(context);
        context.appendAnnotationIntrospector(new AliasIntrospector());
    }
}

JsonMapper mapper = JsonMapper.builder().addModule(new AliasModule()).build();
mapper.readValue("\"rouge\"", Color.class);
// InvalidFormatException; works with insertAnnotationIntrospector()

Expected behavior

Color.RED. The secondary introspector's aliases should be kept for values that have no @JsonAlias. A possible fix is to assign aliasList[i] in JacksonAnnotationIntrospector.findEnumAliases() only when the value has @JsonAlias, and otherwise fill in an empty array only if the slot is still null. With that change, the full 3.x test suite passes.

The open #6210 / #6211 (@JsonAlias with @JsonValue) change EnumResolver, not this method.

Additional context

This issue was found by an LLM-Enhanced Static Analysis Engine and verified manually.