Introduce TestDescriptor.mayRegisterTests()

Fixes #508.

documentation/src/docs/asciidoc/release-notes-5.0.0-M5.adoc

@@ -65,7 +65,8 @@ is placed on the Java 9 module path.
   - `junit-platform-commons`: `ReflectionUtils.findAllClassesInClasspathRoot(Path, Predicate, Predicate)`
 * The `isLeaf()` method of the `org.junit.platform.engine.support.hierarchical.Node`
   interface has been removed.
-* The default methods `prune()` and `pruneTree()` have been removed from `TestDescriptor`.
+* The default methods `prune()`, `pruneTree()`, and `hasTests()` have been removed from
+  `TestDescriptor`.
 * The `DefaultLauncher` no longer prunes trees of `TestDescriptors` it receives from test
   engines. It's the responsibility of test engines to only return `TestDescriptors` they
   wish to execute.
@@ -85,6 +86,8 @@ is placed on the Java 9 module path.
 * When a `TestEngine` is discovered via Java's `ServiceLoader` mechanism, an attempt
   will now be made to determine the location from which the engine class was loaded,
   and if the location URL is available, it will be logged at configuration-level.
+* The `mayRegisterTests()` method may be used to signal that a `TestDescriptor` will
+  register dynamic tests during execution.
 
 
 [[release-notes-5.0.0-m5-junit-jupiter]]

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/TestFactoryTestDescriptor.java

@@ -55,7 +55,7 @@ public Type getType() {
 	}
 
 	@Override
-	public boolean hasTests() {
+	public boolean mayRegisterTests() {
 		return true;
 	}
 

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/TestTemplateTestDescriptor.java

@@ -47,7 +47,7 @@ public Type getType() {
 	}
 
 	@Override
-	public boolean hasTests() {
+	public boolean mayRegisterTests() {
 		return true;
 	}
 

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

@@ -13,6 +13,7 @@
 import static org.junit.platform.commons.meta.API.Usage.Experimental;
 import static org.junit.platform.commons.util.ReflectionUtils.findAllClassesInClasspathRoot;
 import static org.junit.platform.commons.util.ReflectionUtils.findAllClassesInPackage;
+import static org.junit.platform.engine.TestDescriptor.containsTests;
 import static org.junit.platform.engine.support.filter.ClasspathScanningSupport.buildClassNamePredicate;
 
 import java.util.HashSet;
@@ -67,12 +68,11 @@ public void resolveSelectors(EngineDiscoveryRequest request, TestDescriptor engi
 		pruneTree(engineDescriptor);
 	}
 
-	private void pruneTree(TestDescriptor engineDescriptor) {
-		engineDescriptor.accept(testDescriptor -> {
-			if (testDescriptor.isRoot() || testDescriptor.hasTests()) {
-				return;
+	private void pruneTree(TestDescriptor rootDescriptor) {
+		rootDescriptor.accept(testDescriptor -> {
+			if (!testDescriptor.isRoot() && !containsTests(testDescriptor)) {
+				testDescriptor.removeFromHierarchy();
 			}
-			testDescriptor.removeFromHierarchy();
 		});
 	}
 

junit-platform-engine/src/main/java/org/junit/platform/engine/TestDescriptor.java

@@ -185,14 +185,22 @@ default boolean isTest() {
 	}
 
 	/**
-	 * Determine if this descriptor or any of its descendants describes a test.
+	 * Determine if this descriptor may register dynamic tests during execution.
 	 *
-	 * <p>The default implementation returns {@code true} if {@link #isTest()}
-	 * returns {@code true} and otherwise recurses through this descriptor's
-	 * {@linkplain #getChildren() children} to determine if they have tests.
+	 * <p>The default implementation assumes tests are usually known during
+	 * discovery and thus returns {@code false}.
 	 */
-	default boolean hasTests() {
-		return isTest() || getChildren().stream().anyMatch(TestDescriptor::hasTests);
+	default boolean mayRegisterTests() {
+		return false;
+	}
+
+	/**
+	 * Determine if the supplied descriptor or any of its descendants contains
+	 * any tests.
+	 */
+	static boolean containsTests(TestDescriptor testDescriptor) {
+		return testDescriptor.isTest() || testDescriptor.mayRegisterTests()
+				|| testDescriptor.getChildren().stream().anyMatch(TestDescriptor::containsTests);
 	}
 
 	/**

platform-tests/src/test/java/org/junit/platform/engine/support/hierarchical/DemoHierarchicalContainerDescriptor.java

@@ -47,7 +47,7 @@ public Type getType() {
 	}
 
 	@Override
-	public boolean hasTests() {
+	public boolean mayRegisterTests() {
 		return true;
 	}