Review comment to allow negation of unrestricted

api/src/main/java/jakarta/data/metamodel/restrict/Unmatchable.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2025 Contributors to the Eclipse Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package jakarta.data.metamodel.restrict;
+
+import java.util.List;
+
+class Unmatchable<T> implements CompositeRestriction<T> {
+    static final Unmatchable<?> INSTANCE = new Unmatchable<>();
+
+    // prevent instantiation by others
+    private Unmatchable() {
+    }
+
+    @Override
+    public boolean isNegated() {
+        return false;
+    }
+
+    @Override
+    @SuppressWarnings("unchecked")
+    public CompositeRestriction<T> negate() {
+        return (CompositeRestriction<T>) Unrestricted.INSTANCE;
+    }
+
+    @Override
+    public List<Restriction<T>> restrictions() {
+        return List.of();
+    }
+
+    @Override
+    public Type type() {
+        return Type.ANY;
+    }
+}

api/src/main/java/jakarta/data/metamodel/restrict/Unrestricted.java

@@ -32,9 +32,9 @@ public boolean isNegated() {
     }
 
     @Override
+    @SuppressWarnings("unchecked")
     public CompositeRestriction<T> negate() {
-        throw new UnsupportedOperationException(
-                "The absence of restrictions cannot be negated.");
+        return (CompositeRestriction<T>) Unmatchable.INSTANCE;
     }
 
     @Override

api/src/test/java/jakarta/data/metamodel/restrict/RestrictTest.java

@@ -171,6 +171,23 @@ void shouldCreateNegatedEndsWithRestriction() {
         });
     }
 
+    @Test
+    void shouldCreateUnmatchableRestrictionWhenNegatingUnrestricted() {
+        Restriction<Employee> restriction = Restrict.unrestricted();
+        Restriction<Employee> negated = restriction.negate();
+
+        assertThat(negated).isInstanceOf(Unmatchable.class);
+
+        Unmatchable<Employee> unmatchable = (Unmatchable<Employee>) negated;
+
+        SoftAssertions.assertSoftly(soft -> {
+            soft.assertThat(unmatchable.isNegated()).isEqualTo(false);
+            soft.assertThat(unmatchable.negate()).isEqualTo(restriction);
+            soft.assertThat(unmatchable.restrictions()).isEmpty();
+            soft.assertThat(unmatchable.type()).isEqualTo(CompositeRestriction.Type.ANY);
+        });
+    }
+
     @Test
     void shouldCreateUnrestricted() {
         Restriction<Employee> restriction = Restrict.unrestricted();
@@ -205,12 +222,4 @@ void shouldThrowExceptionForInvalidWildcard() {
                 .isInstanceOf(IllegalArgumentException.class)
                 .hasMessage("Cannot use the same character (_) for both types of wildcards.");
     }
-
-    @Test
-    void shouldThrowExceptionForNegatingUnrestrictedRestriction() {
-        Restriction<Employee> unrestricted = Restrict.unrestricted();
-        assertThatThrownBy(() -> unrestricted.negate())
-                .isInstanceOf(UnsupportedOperationException.class)
-                .hasMessage("The absence of restrictions cannot be negated.");
-    }
 }