@JsonIgnore
    public boolean canHaveCustomFieldMappings() {
        return !this.indexTemplateType().map(TEMPLATE_TYPES_FOR_INDEX_SETS_WITH_IMMUTABLE_FIELD_TYPES::contains).orElse(false);
    }
@Test
    public void testIlluminateIndexWithChangedFieldMappingsIsIllegal() {
        assertFalse(testIndexSetConfig(IndexTemplateProvider.ILLUMINATE_INDEX_TEMPLATE_TYPE,
                new CustomFieldMappings(List.of(new CustomFieldMapping("john", "long"))),
                null).canHaveCustomFieldMappings());
    }