Avoid reporting discovery warnings for abstract inner classes with tests

Resolves #4635.

Author: marcphilipp

documentation/src/docs/asciidoc/release-notes/release-notes-5.13.2.adoc

@@ -37,6 +37,8 @@ repository on GitHub.
 
 * Stop reporting discovery issues for cyclic inner class hierarchies not annotated with
   `@Nested`.
+* Stop reporting discovery issues for _abstract_ inner classes that contain test methods
+  but are not annotated with `@Nested`.
 
 [[release-notes-5.13.2-junit-jupiter-deprecations-and-breaking-changes]]
 ==== Deprecations and Breaking Changes

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/discovery/ClassSelectorResolver.java

@@ -19,6 +19,7 @@
 import static org.junit.platform.commons.support.ReflectionSupport.findMethods;
 import static org.junit.platform.commons.util.FunctionUtils.where;
 import static org.junit.platform.commons.util.ReflectionUtils.isInnerClass;
+import static org.junit.platform.commons.util.ReflectionUtils.isNotAbstract;
 import static org.junit.platform.commons.util.ReflectionUtils.streamNestedClasses;
 import static org.junit.platform.engine.discovery.DiscoverySelectors.selectUniqueId;
 import static org.junit.platform.engine.support.discovery.SelectorResolver.Resolution.unresolved;
@@ -113,7 +114,8 @@ public Resolution resolve(NestedClassSelector selector, Context context) {
 					parent -> Optional.of(newMemberClassTestDescriptor(parent, nestedClass))));
 			}
 		}
-		else if (isInnerClass(nestedClass) && predicates.looksLikeIntendedTestClass(nestedClass)) {
+		else if (isInnerClass(nestedClass) && isNotAbstract(nestedClass)
+				&& predicates.looksLikeIntendedTestClass(nestedClass)) {
 			String message = "Inner class '%s' looks like it was intended to be a test class but will not be executed. It must be static or annotated with @Nested.".formatted(
 				nestedClass.getName());
 			issueReporter.reportIssue(DiscoveryIssue.builder(Severity.WARNING, message) //

jupiter-tests/src/test/java/org/junit/jupiter/engine/NestedTestClassesTests.java

@@ -230,6 +230,13 @@ void individualMethodsWithinRecursiveNestedTestClassHierarchiesAreExecuted() {
 		executionResults.testEvents().assertStatistics(stats -> stats.started(1).succeeded(1));
 	}
 
+	@Test
+	void doesNotReportDiscoveryIssueForAbstractInnerClass() {
+		var discoveryIssues = discoverTestsForClass(ConcreteWithExtendedInnerClassTestCase.class).getDiscoveryIssues();
+
+		assertThat(discoveryIssues).isEmpty();
+	}
+
 	private void assertNestedCycle(Class<?> start, Class<?> from, Class<?> to) {
 		var results = executeTestsForClass(start);
 		var expectedMessage = "Cause: org.junit.platform.commons.JUnitException: Detected cycle in inner class hierarchy between %s and %s".formatted(
@@ -420,4 +427,19 @@ void nested() {
 		}
 	}
 
+	static class AbstractBaseWithInnerClassTestCase {
+		@SuppressWarnings("InnerClassMayBeStatic")
+		abstract class AbstractInnerClass {
+			@Test
+			void test() {
+			}
+		}
+	}
+
+	static class ConcreteWithExtendedInnerClassTestCase extends AbstractBaseWithInnerClassTestCase {
+		@Nested
+		class NestedTests extends AbstractInnerClass {
+		}
+	}
+
 }