GROOVY-12188: accept covariant override of abstract method regardless…

src/main/java/org/codehaus/groovy/classgen/ClassCompletionVerifier.java

@@ -240,6 +240,14 @@ private void checkNoAbstractMethodsNonAbstractClass(final ClassNode node) {
                 what = "Can't have an abstract method in a non-abstract class." +
                         " The " + getDescription(node) + " must be declared abstract or" +
                         " the " + getDescription(method) + " must be implemented.";
+            } else if (!sameArgsMethod.isAbstract() && !sameArgsMethod.isStatic()
+                    && !ClassHelper.isPrimitiveType(sameArgsMethod.getReturnType())
+                    && (sameArgsMethod.getReturnType().isDerivedFrom(method.getReturnType())
+                        || sameArgsMethod.getReturnType().implementsInterface(method.getReturnType()))) {
+                // GROOVY-12188: implemented with covariant return type; cannot rely on the
+                // bridge method here, since Verifier may not have processed the declaring
+                // class yet -- GROOVY-10687 emits a nest host after its member classes
+                continue;
             } else {
                 what = "Abstract " + getDescription(method) + " is not implemented but a " +
                         "method of the same name but different return type is defined: " +

src/test/groovy/bugs/Groovy12188.groovy

@@ -0,0 +1,102 @@
+/*
+ *  Licensed to the Apache Software Foundation (ASF) under one
+ *  or more contributor license agreements.  See the NOTICE file
+ *  distributed with this work for additional information
+ *  regarding copyright ownership.  The ASF licenses this file
+ *  to you 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.
+ */
+package bugs
+
+import org.junit.jupiter.api.Test
+
+import static groovy.test.GroovyAssert.assertScript
+import static groovy.test.GroovyAssert.shouldFail
+
+/**
+ * A generic abstract method implemented with a covariant (narrowed) return type is
+ * only recognised as implemented by {@code ClassCompletionVerifier} once the class
+ * declaring the override has been through {@code Verifier}, which adds the bridge
+ * method whose erased descriptor matches the abstract method's. Since the
+ * GROOVY-10687 fix, a top-level class whose inner class contains a closure is
+ * generated after its nest members — which can place it after its own subclass, so
+ * the subclass is verified before the bridge exists and compilation fails with
+ * "Abstract method 'T someValue()' is not implemented but a method of the same name
+ * but different return type is defined" (GROOVY-12188). These tests pin that the
+ * check recognises a covariant override independent of class generation order.
+ */
+final class Groovy12188 {
+
+    // the reporter's example: the nest host with an inner-class closure sorts after its subclass
+    @Test
+    void testCovariantOverrideWithNestedClassClosure() {
+        assertScript '''
+            abstract class ProviderSpec<T> {
+                abstract T someValue()
+            }
+
+            abstract class CollectionPropertySpec<C extends Collection<String>> extends ProviderSpec<C> {
+                C someValue() { return null }
+
+                static class Nested<T> {
+                    void trigger(Optional<T> p) { p.map { it } }
+                }
+            }
+
+            class DefaultListPropertyTest extends CollectionPropertySpec<List<String>> {
+            }
+
+            assert new DefaultListPropertyTest().someValue() == null
+        '''
+    }
+
+    // no ordering can help here: the nest host must follow its member for the
+    // NestMembers attribute but precede it for the bridge-based abstract check
+    @Test
+    void testCovariantOverrideWithNestMemberSubclass() {
+        assertScript '''
+            abstract class ProviderSpec<T> {
+                abstract T someValue()
+            }
+
+            abstract class CollectionPropertySpec<C extends Collection<String>> extends ProviderSpec<C> {
+                C someValue() { return null }
+
+                static class Inner extends CollectionPropertySpec<List<String>> {
+                    def c = { it }
+                }
+            }
+
+            assert new CollectionPropertySpec.Inner().someValue() == null
+        '''
+    }
+
+    // a same-name method whose return type is not covariant must still be rejected
+    @Test
+    void testNonCovariantReturnTypeStillRejected() {
+        def err = shouldFail '''
+            abstract class A<T extends Number> {
+                abstract T someValue()
+            }
+
+            class B extends A<Integer> {
+                String someValue() { return null }
+
+                static class Nested {
+                    def c = { it }
+                }
+            }
+        '''
+        assert err.message.contains('someValue')
+    }
+}