Introduce zero invocations in test templates and parameterized tests

Add a flag to ParameterizedTest to control arguments requirement. This
allows users to explicitly opt out from validation of arguments set
count and silently skip a test if no arguments are provided

In general, support TestTemplateInvocationContextProvider returning zero
invocation contexts. Such providers must override new interface method
to indicate that the framework should expect "no context returned"

Resolves junit-team#1477

junit-jupiter-api/src/main/java/org/junit/jupiter/api/extension/TestTemplateInvocationContextProvider.java

@@ -10,6 +10,7 @@
 
 package org.junit.jupiter.api.extension;
 
+import static org.apiguardian.api.API.Status.EXPERIMENTAL;
 import static org.apiguardian.api.API.Status.STABLE;
 
 import java.util.stream.Stream;
@@ -86,4 +87,22 @@ public interface TestTemplateInvocationContextProvider extends Extension {
 	 */
 	Stream<TestTemplateInvocationContext> provideTestTemplateInvocationContexts(ExtensionContext context);
 
+	/**
+	 * Signals that in the given {@linkplain ExtensionContext extension context} the provider may return zero
+	 * {@linkplain TestTemplateInvocationContext invocation contexts}
+	 *
+	 * <p>If provider returns empty stream from {@link #provideTestTemplateInvocationContexts(ExtensionContext)}
+	 * this will be considered an execution error. Override this method to ignore the absence of invocation contexts.
+	 *
+	 * @param extensionContext the extension context for the test template method about
+	 * to be invoked; never {@code null}
+	 * @return {@code true} to allow zero contexts, {@code false} (default) to fail execution in case of zero contexts.
+	 *
+	 * @since 5.11
+	 */
+	@API(status = EXPERIMENTAL, since = "5.11")
+	default boolean mayReturnEmptyInvocationContext(ExtensionContext extensionContext) {
+		return false;
+	}
+
 }

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

@@ -109,7 +109,11 @@ public JupiterEngineExecutionContext execute(JupiterEngineExecutionContext conte
 				.map(Optional::get)
 				.forEach(invocationTestDescriptor -> execute(dynamicTestExecutor, invocationTestDescriptor));
 		// @formatter:on
-		validateWasAtLeastInvokedOnce(invocationIndex.get(), providers);
+		boolean allProvidersMayBeEmpty = providers.stream().allMatch(
+			p -> p.mayReturnEmptyInvocationContext(extensionContext));
+		if (!allProvidersMayBeEmpty) {
+			validateWasAtLeastInvokedOnce(invocationIndex.get(), extensionContext, providers);
+		}
 		return context;
 	}
 
@@ -142,13 +146,16 @@ private void execute(DynamicTestExecutor dynamicTestExecutor, TestDescriptor tes
 		dynamicTestExecutor.execute(testDescriptor);
 	}
 
-	private void validateWasAtLeastInvokedOnce(int invocationIndex,
+	private void validateWasAtLeastInvokedOnce(int invocationIndex, ExtensionContext extensionContext,
 			List<TestTemplateInvocationContextProvider> providers) {
 
-		Preconditions.condition(invocationIndex > 0,
+		boolean allMayReturnEmptyContext = providers.stream().allMatch(
+			p -> p.mayReturnEmptyInvocationContext(extensionContext));
+
+		Preconditions.condition(invocationIndex > 0 || allMayReturnEmptyContext,
 			() -> "None of the supporting " + TestTemplateInvocationContextProvider.class.getSimpleName() + "s "
-					+ providers.stream().map(provider -> provider.getClass().getSimpleName()).collect(
-						joining(", ", "[", "]"))
+					+ providers.stream().filter(p -> !p.mayReturnEmptyInvocationContext(extensionContext)).map(
+						provider -> provider.getClass().getSimpleName()).collect(joining(", ", "[", "]"))
 					+ " provided a non-empty stream");
 	}
 

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/TestTemplateInvocationTests.java

@@ -358,6 +358,35 @@ void templateWithSupportingProviderButNoInvocationsReportsFailure() {
 							+ "] provided a non-empty stream")))));
 	}
 
+	@Test
+	void templateWithSupportingProviderAllowingNoInvocationsDoesNotFail() {
+		LauncherDiscoveryRequest request = request().selectors(
+			selectMethod(MyTestTemplateTestCase.class, "templateWithSupportingProviderAllowingNoInvocations")).build();
+
+		EngineExecutionResults executionResults = executeTests(request);
+
+		executionResults.allEvents().assertEventsMatchExactly( //
+			wrappedInContainerEvents(MyTestTemplateTestCase.class,
+				event(container("templateWithSupportingProviderAllowingNoInvocations"), started()),
+				event(container("templateWithSupportingProviderAllowingNoInvocations"), finishedSuccessfully())));
+	}
+
+	@Test
+	void templateWithMixedProvidersNoInvocationReportsFailure() {
+		LauncherDiscoveryRequest request = request().selectors(selectMethod(MyTestTemplateTestCase.class,
+			"templateWithMultipleProvidersAllowingAndRestrictingToProvideNothing")).build();
+
+		EngineExecutionResults executionResults = executeTests(request);
+
+		executionResults.allEvents().assertEventsMatchExactly( //
+			wrappedInContainerEvents(MyTestTemplateTestCase.class, //
+				event(container("templateWithMultipleProvidersAllowingAndRestrictingToProvideNothing"), started()), //
+				event(container("templateWithMultipleProvidersAllowingAndRestrictingToProvideNothing"),
+					finishedWithFailure(message("None of the supporting TestTemplateInvocationContextProviders ["
+							+ InvocationContextProviderThatSupportsEverythingButProvidesNothing.class.getSimpleName()
+							+ "] provided a non-empty stream")))));
+	}
+
 	@Test
 	void templateWithCloseableStream() {
 		LauncherDiscoveryRequest request = request().selectors(
@@ -470,6 +499,19 @@ void templateWithSupportingProviderButNoInvocations() {
 			fail("never called");
 		}
 
+		@ExtendWith(InvocationContextProviderThatSupportsEverythingAllowsProvideNothing.class)
+		@TestTemplate
+		void templateWithSupportingProviderAllowingNoInvocations() {
+			fail("never called");
+		}
+
+		@ExtendWith(InvocationContextProviderThatSupportsEverythingButProvidesNothing.class)
+		@ExtendWith(InvocationContextProviderThatSupportsEverythingAllowsProvideNothing.class)
+		@TestTemplate
+		void templateWithMultipleProvidersAllowingAndRestrictingToProvideNothing() {
+			fail("never called");
+		}
+
 		@ExtendWith(InvocationContextProviderWithCloseableStream.class)
 		@TestTemplate
 		void templateWithCloseableStream() {
@@ -769,6 +811,25 @@ public Stream<TestTemplateInvocationContext> provideTestTemplateInvocationContex
 		}
 	}
 
+	private static class InvocationContextProviderThatSupportsEverythingAllowsProvideNothing
+			implements TestTemplateInvocationContextProvider {
+
+		@Override
+		public boolean supportsTestTemplate(ExtensionContext context) {
+			return true;
+		}
+
+		@Override
+		public Stream<TestTemplateInvocationContext> provideTestTemplateInvocationContexts(ExtensionContext context) {
+			return Stream.empty();
+		}
+
+		@Override
+		public boolean mayReturnEmptyInvocationContext(ExtensionContext extensionContext) {
+			return true;
+		}
+	}
+
 	private static class InvocationContextProviderWithCloseableStream implements TestTemplateInvocationContextProvider {
 
 		private static AtomicBoolean streamClosed = new AtomicBoolean(false);

junit-jupiter-params/src/main/java/org/junit/jupiter/params/ParameterizedTest.java

@@ -291,4 +291,17 @@
 	@API(status = STABLE, since = "5.10")
 	boolean autoCloseArguments() default true;
 
+	/**
+	 * Configure whether a test requires at least one set of arguments.
+	 * Override this value if the absence of arguments is expected in some cases and
+	 * should not cause the test framework errors.
+	 *
+	 * <p>Defaults to {@code true}.
+	 *
+	 * <p></p>
+	 * @since 5.11
+	 */
+	@API(status = EXPERIMENTAL, since = "5.11")
+	boolean requireArguments() default true;
+
 }

junit-jupiter-params/src/main/java/org/junit/jupiter/params/ParameterizedTestExtension.java

@@ -79,6 +79,8 @@ public Stream<TestTemplateInvocationContext> provideTestTemplateInvocationContex
 		ParameterizedTestNameFormatter formatter = createNameFormatter(extensionContext, templateMethod, methodContext,
 			displayName, argumentMaxLength);
 		AtomicLong invocationCount = new AtomicLong(0);
+		ParameterizedTest parameterizedTest = findAnnotation(templateMethod, ParameterizedTest.class).get();
+		boolean ignoreZeroInvocations = !parameterizedTest.requireArguments();
 
 		// @formatter:off
 		return findRepeatableAnnotations(templateMethod, ArgumentsSource.class)
@@ -92,11 +94,19 @@ public Stream<TestTemplateInvocationContext> provideTestTemplateInvocationContex
 					return createInvocationContext(formatter, methodContext, arguments, invocationCount.intValue());
 				})
 				.onClose(() ->
-						Preconditions.condition(invocationCount.get() > 0,
+						Preconditions.condition(invocationCount.get() > 0 || ignoreZeroInvocations,
 								"Configuration error: You must configure at least one set of arguments for this @ParameterizedTest"));
 		// @formatter:on
 	}
 
+	@Override
+	public boolean mayReturnEmptyInvocationContext(ExtensionContext extensionContext) {
+		Method templateMethod = extensionContext.getRequiredTestMethod();
+		return findAnnotation(templateMethod, ParameterizedTest.class).map(
+			parameterizedTest -> !parameterizedTest.requireArguments()).orElse(
+				TestTemplateInvocationContextProvider.super.mayReturnEmptyInvocationContext(extensionContext));
+	}
+
 	@SuppressWarnings("ConstantConditions")
 	private ArgumentsProvider instantiateArgumentsProvider(Class<? extends ArgumentsProvider> clazz) {
 		try {

junit-jupiter-params/src/test/java/org/junit/jupiter/params/ParameterizedTestExtensionTests.java

@@ -140,6 +140,18 @@ void throwsExceptionWhenParameterizedTestIsNotInvokedAtLeastOnce() {
 			"Configuration error: You must configure at least one set of arguments for this @ParameterizedTest");
 	}
 
+	@Test
+	void doesNotThrowExceptionWhenParametrizedTestDoesNotRequireArguments() {
+		var extensionContextWithAnnotatedTestMethod = getExtensionContextReturningSingleMethod(
+			new TestCaseAllowNoArgumentsMethod());
+
+		var stream = this.parameterizedTestExtension.provideTestTemplateInvocationContexts(
+			extensionContextWithAnnotatedTestMethod);
+		// cause the stream to be evaluated
+		stream.toArray();
+		stream.close();
+	}
+
 	@Test
 	void throwsExceptionWhenArgumentsProviderIsNotStatic() {
 		var extensionContextWithAnnotatedTestMethod = getExtensionContextReturningSingleMethod(
@@ -298,6 +310,13 @@ void method() {
 		}
 	}
 
+	static class TestCaseAllowNoArgumentsMethod {
+
+		@ParameterizedTest(requireArguments = false)
+		void method() {
+		}
+	}
+
 	static class ArgumentsProviderWithCloseHandlerTestCase {
 
 		@ParameterizedTest

junit-jupiter-params/src/test/java/org/junit/jupiter/params/ParameterizedTestIntegrationTests.java

@@ -30,6 +30,7 @@
 import static org.junit.platform.testkit.engine.EventConditions.abortedWithReason;
 import static org.junit.platform.testkit.engine.EventConditions.container;
 import static org.junit.platform.testkit.engine.EventConditions.displayName;
+import static org.junit.platform.testkit.engine.EventConditions.engine;
 import static org.junit.platform.testkit.engine.EventConditions.event;
 import static org.junit.platform.testkit.engine.EventConditions.finishedSuccessfully;
 import static org.junit.platform.testkit.engine.EventConditions.finishedWithFailure;
@@ -444,6 +445,24 @@ void displayNamePatternFromConfiguration() {
 				.haveExactly(1, event(displayName("2"), started()));
 	}
 
+	@Test
+	void failsWhenArgumentsRequiredButNoneProvided() {
+		var result = execute(ZeroArgumentsTestCase.class, "testThatRequiresArguments", String.class);
+		result.containerEvents().assertThatEvents().haveExactly(1, event(finishedWithFailure(message(
+			"Configuration error: You must configure at least one set of arguments for this @ParameterizedTest"))));
+	}
+
+	@Test
+	void failsWhenArgumentsAreNotRequiredAndNoneProvided() {
+		var result = execute(ZeroArgumentsTestCase.class, "testThatDoesNotRequireArguments", String.class);
+		result.allEvents().assertEventsMatchExactly( //
+			event(engine(), started()), event(container(ZeroArgumentsTestCase.class), started()),
+			event(container("testThatDoesNotRequireArguments"), started()),
+			event(container("testThatDoesNotRequireArguments"), finishedSuccessfully()),
+			event(container(ZeroArgumentsTestCase.class), finishedSuccessfully()),
+			event(engine(), finishedSuccessfully()));
+	}
+
 	private EngineExecutionResults execute(DiscoverySelector... selectors) {
 		return EngineTestKit.engine(new JupiterTestEngine()).selectors(selectors).execute();
 	}
@@ -2119,6 +2138,25 @@ void testWithRepeatableArgumentsSource(String argument) {
 		}
 	}
 
+	static class ZeroArgumentsTestCase {
+
+		@ParameterizedTest
+		@MethodSource("zeroArgumentsProvider")
+		void testThatRequiresArguments(String argument) {
+			fail("This test should not be executed, because no arguments are provided.");
+		}
+
+		@ParameterizedTest(requireArguments = false)
+		@MethodSource("zeroArgumentsProvider")
+		void testThatDoesNotRequireArguments(String argument) {
+			fail("This test should not be executed, because no arguments are provided.");
+		}
+
+		public static Stream<Arguments> zeroArgumentsProvider() {
+			return Stream.empty();
+		}
+	}
+
 	private static class TwoSingleStringArgumentsProvider implements ArgumentsProvider {
 
 		@Override