WRITE_ENUMS_TO_LOWERCASE must use Locale.ROOT for case folding

src/test/java/tools/jackson/databind/ser/enums/EnumSerializationTest.java

@@ -3,6 +3,8 @@
 import java.util.*;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.parallel.ResourceLock;
+import org.junit.jupiter.api.parallel.Resources;
 
 import com.fasterxml.jackson.annotation.*;
 
@@ -91,6 +93,10 @@ private LowerCaseEnum() { }
         public String toString() { return name().toLowerCase(Locale.ROOT); }
     }
 
+    protected static enum LocaleSensitiveEnum {
+        IS_ADMIN
+    }
+
     static class MapBean {
         public Map<TestEnum,Integer> map = new HashMap<>();
 
@@ -392,6 +398,26 @@ public void testEnumFeature_WRITE_ENUMS_TO_LOWERCASE() throws Exception {
         assertEquals(q("b"), m.writeValueAsString(TestEnum.B));
         // NOTE: cannot be dynamically changed
     }
+
+    // [databind#5993]: Follow-up to #5994; enum lower-case serialization
+    // should not use the default Locale
+    @ResourceLock(Resources.LOCALE)
+    @Test
+    public void testEnumFeature_WRITE_ENUMS_TO_LOWERCASEUsesRootLocale() throws Exception {
+        Locale old = Locale.getDefault();
+        try {
+            Locale.setDefault(Locale.forLanguageTag("tr-TR"));
+            assertEquals("\u0131", "I".toLowerCase(),
+                    "Test requires a default locale where \"I\".toLowerCase() yields U+0131");
+
+            ObjectMapper m = jsonMapperBuilder()
+                    .configure(EnumFeature.WRITE_ENUMS_TO_LOWERCASE, true)
+                    .build();
+            assertEquals(q("is_admin"), m.writeValueAsString(LocaleSensitiveEnum.IS_ADMIN));
+        } finally {
+            Locale.setDefault(old);
+        }
+    }
 }
 
 // [JACKSON-757], non-inner enum

src/test/java/tools/jackson/databind/util/EnumValuesTest.java

@@ -1,8 +1,11 @@
 package tools.jackson.databind.util;
 
 import java.util.List;
+import java.util.Locale;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.parallel.ResourceLock;
+import org.junit.jupiter.api.parallel.Resources;
 
 import tools.jackson.databind.ObjectMapper;
 import tools.jackson.databind.SerializationConfig;
@@ -30,6 +33,10 @@ enum ABC {
         public String toString() { return desc; }
     }
 
+    enum LocaleSensitiveABC {
+        IS_ADMIN
+    }
+
     private final ObjectMapper MAPPER = newJsonMapper();
 
     @Test
@@ -91,6 +98,28 @@ public void testConstructFromNameLowerCased() {
         assertEquals(3, values.internalMap().size());
     }
 
+    // [databind#5993]: Follow-up to #5994; deprecated EnumValues lower-case path
+    // should not use the default Locale
+    @ResourceLock(Resources.LOCALE)
+    @Test
+    public void testConstructFromNameLowerCasedWithRootLocale() {
+        Locale old = Locale.getDefault();
+        try {
+            Locale.setDefault(Locale.forLanguageTag("tr-TR"));
+            assertEquals("\u0131", "I".toLowerCase(),
+                    "Test requires a default locale where \"I\".toLowerCase() yields U+0131");
+
+            SerializationConfig cfg = MAPPER.serializationConfig()
+                    .with(EnumFeature.WRITE_ENUMS_TO_LOWERCASE);
+            AnnotatedClass enumClass = resolve(MAPPER, LocaleSensitiveABC.class);
+
+            EnumValues values = EnumValues.constructFromName(cfg, enumClass);
+            assertEquals("is_admin", values.serializedValueFor(LocaleSensitiveABC.IS_ADMIN).toString());
+        } finally {
+            Locale.setDefault(old);
+        }
+    }
+
     private AnnotatedClass resolve(ObjectMapper mapper, Class<?> enumClass) {
         return AnnotatedClassResolver.resolve(mapper.serializationConfig(),
                 mapper.constructType(enumClass), null);