Extension of to stream converter method.

Examples of benefits:

- Kotlin Sequence support for @testfactory
- Kotlin Sequence support for @MethodSource

Classes that expose an Iterator returning method, can be converted to a stream.

```markdown
---
I hereby agree to the terms of the JUnit Contributor License Agreement.
```

junit-jupiter-engine/src/test/kotlin/org/junit/jupiter/api/KotlinDynamicTests.kt

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2015-2023 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+package org.junit.jupiter.api
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.DynamicTest.dynamicTest
+import java.math.BigDecimal
+import java.math.BigDecimal.ONE
+import java.math.MathContext
+import java.math.BigInteger as BigInt
+import java.math.RoundingMode as Rounding
+
+/**
+ * Unit tests for JUnit Jupiter [TestFactory] use in kotlin classes.
+ *
+ * @since 5.10
+ */
+class KotlinDynamicTests {
+
+    @Nested
+    inner class SequenceReturningTestFactoryTests {
+
+        @TestFactory
+        fun `Dynamic tests returned as Kotlin sequence`() = generateSequence(0) { it + 2 }
+            .map { dynamicTest("$it should be even") { assertTrue(it % 2 == 0) } }
+            .take(10)
+
+        @TestFactory
+        fun `Consecutive fibonacci nr ratios, should converge to golden ratio as n increases`(): Sequence<DynamicTest> {
+            val scale = 5
+            val goldenRatio = (ONE + 5.toBigDecimal().sqrt(MathContext(scale + 10, Rounding.HALF_UP)))
+                .divide(2.toBigDecimal(), scale, Rounding.HALF_UP)
+
+            fun shouldApproximateGoldenRatio(cur: BigDecimal, next: BigDecimal) =
+                next.divide(cur, scale, Rounding.HALF_UP).let {
+                    dynamicTest("$cur / $next = $it should approximate the golden ratio in $scale decimals") {
+                        assertEquals(goldenRatio, it)
+                    }
+                }
+            return generateSequence(BigInt.ONE to BigInt.ONE) { (cur, next) -> next to cur + next }
+                .map { (cur) -> cur.toBigDecimal() }
+                .zipWithNext(::shouldApproximateGoldenRatio)
+                .drop(14)
+                .take(10)
+        }
+    }
+}

junit-jupiter-params/src/test/kotlin/org/junit/jupiter/params/aggregator/KotlinParameterizedTests.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2015-2023 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+package org.junit.jupiter.params.aggregator
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.Arguments.arguments
+import org.junit.jupiter.params.provider.MethodSource
+import java.time.Month
+
+/**
+ * Tests for ParameterizedTest kotlin compatibility
+ */
+object KotlinParameterizedTests {
+
+    @ParameterizedTest
+    @MethodSource("dataProvidedByKotlinSequence")
+    fun `a method source can be supplied by a Sequence returning method`(value: Int, month: Month) {
+        assertEquals(value, month.value)
+    }
+
+    @JvmStatic
+    private fun dataProvidedByKotlinSequence() = sequenceOf(
+        arguments(1, Month.JANUARY),
+        arguments(3, Month.MARCH),
+        arguments(8, Month.AUGUST),
+        arguments(5, Month.MAY),
+        arguments(12, Month.DECEMBER)
+    )
+}

junit-platform-commons/src/main/java/org/junit/platform/commons/util/CollectionUtils.java

@@ -18,6 +18,7 @@
 import static org.apiguardian.api.API.Status.INTERNAL;
 
 import java.lang.reflect.Array;
+import java.lang.reflect.Method;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -26,6 +27,7 @@
 import java.util.List;
 import java.util.ListIterator;
 import java.util.Set;
+import java.util.Spliterator;
 import java.util.function.Consumer;
 import java.util.stream.Collector;
 import java.util.stream.DoubleStream;
@@ -34,7 +36,9 @@
 import java.util.stream.Stream;
 
 import org.apiguardian.api.API;
+import org.junit.platform.commons.JUnitException;
 import org.junit.platform.commons.PreconditionViolationException;
+import org.junit.platform.commons.function.Try;
 
 /**
  * Collection of utilities for working with {@link Collection Collections}.
@@ -99,7 +103,7 @@ public static <T> Set<T> toSet(T[] values) {
 	 * returned, so if more control over the returned list is required,
 	 * consider creating a new {@code Collector} implementation like the
 	 * following:
-	 *
+	 * <p>
 	 * <pre class="code">
 	 * public static &lt;T&gt; Collector&lt;T, ?, List&lt;T&gt;&gt; toUnmodifiableList(Supplier&lt;List&lt;T&gt;&gt; listSupplier) {
 	 *     return Collectors.collectingAndThen(Collectors.toCollection(listSupplier), Collections::unmodifiableList);
@@ -139,7 +143,11 @@ public static boolean isConvertibleToStream(Class<?> type) {
 				|| Iterable.class.isAssignableFrom(type)//
 				|| Iterator.class.isAssignableFrom(type)//
 				|| Object[].class.isAssignableFrom(type)//
-				|| (type.isArray() && type.getComponentType().isPrimitive()));
+				|| (type.isArray() && type.getComponentType().isPrimitive())//
+				|| Arrays.stream(type.getMethods())//
+						.filter(m -> m.getName().equals("iterator"))//
+						.map(Method::getReturnType)//
+						.anyMatch(returnType -> returnType == Iterator.class));
 	}
 
 	/**
@@ -155,6 +163,7 @@ public static boolean isConvertibleToStream(Class<?> type) {
 	 * <li>{@link Iterator}</li>
 	 * <li>{@link Object} array</li>
 	 * <li>primitive array</li>
+	 * <li>An object that contains a method with name `iterator` returning an Iterator object</li>
 	 * </ul>
 	 *
 	 * @param object the object to convert into a stream; never {@code null}
@@ -201,8 +210,31 @@ public static Stream<?> toStream(Object object) {
 		if (object.getClass().isArray() && object.getClass().getComponentType().isPrimitive()) {
 			return IntStream.range(0, Array.getLength(object)).mapToObj(i -> Array.get(object, i));
 		}
-		throw new PreconditionViolationException(
-			"Cannot convert instance of " + object.getClass().getName() + " into a Stream: " + object);
+		return tryConvertToStreamByReflection(object);
+	}
+
+	private static Stream<?> tryConvertToStreamByReflection(Object object) {
+		Preconditions.notNull(object, "Object must not be null");
+		try {
+			String name = "iterator";
+			Method method = object.getClass().getMethod(name);
+			if (method.getReturnType() == Iterator.class) {
+				return stream(() -> tryIteratorToSpliterator(object, method), ORDERED, false);
+			}
+			else {
+				throw new PreconditionViolationException(
+					"Method with name 'iterator' does not return " + Iterator.class.getName());
+			}
+		}
+		catch (NoSuchMethodException | IllegalStateException e) {
+			throw new PreconditionViolationException(//
+				"Cannot convert instance of " + object.getClass().getName() + " into a Stream: " + object, e);
+		}
+	}
+
+	private static Spliterator<?> tryIteratorToSpliterator(Object object, Method method) {
+		return Try.call(() -> spliteratorUnknownSize((Iterator<?>) method.invoke(object), ORDERED))//
+				.getOrThrow(e -> new JUnitException("Cannot invoke method " + method.getName() + " onto " + object, e));//
 	}
 
 	/**

platform-tests/src/test/java/org/junit/platform/commons/util/CollectionUtilsTests.java

@@ -21,10 +21,13 @@
 
 import java.lang.reflect.Array;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Set;
+import java.util.Spliterator;
+import java.util.Spliterators;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.stream.DoubleStream;
 import java.util.stream.IntStream;
@@ -94,6 +97,7 @@ void toUnmodifiableListThrowsOnMutation() {
 			Collection.class, //
 			Iterable.class, //
 			Iterator.class, //
+			IteratorProvider.class, //
 			Object[].class, //
 			String[].class, //
 			int[].class, //
@@ -115,10 +119,11 @@ static Stream<Object> objectsConvertibleToStreams() {
 			Stream.of("cat", "dog"), //
 			DoubleStream.of(42.3), //
 			IntStream.of(99), //
-			LongStream.of(100000000), //
+			LongStream.of(100_000_000), //
 			Set.of(1, 2, 3), //
 			Arguments.of((Object) new Object[] { 9, 8, 7 }), //
-			new int[] { 5, 10, 15 }//
+			new int[] { 5, 10, 15 }, //
+			IteratorProvider.of(new Integer[] { 1, 2, 3, 4, 5 })//
 		);
 	}
 
@@ -129,6 +134,8 @@ static Stream<Object> objectsConvertibleToStreams() {
 			Object.class, //
 			Integer.class, //
 			String.class, //
+			IteratorProviderNotUsable.class, //
+			Spliterator.class, //
 			int.class, //
 			boolean.class //
 	})
@@ -196,7 +203,7 @@ void toStreamWithLongStream() {
 	}
 
 	@Test
-	@SuppressWarnings({ "unchecked", "serial" })
+	@SuppressWarnings({ "unchecked" })
 	void toStreamWithCollection() {
 		var collectionStreamClosed = new AtomicBoolean(false);
 		Collection<String> input = new ArrayList<>() {
@@ -241,6 +248,24 @@ void toStreamWithIterator() {
 		assertThat(result).containsExactly("foo", "bar");
 	}
 
+	@Test
+	@SuppressWarnings("unchecked")
+	void toStreamWithIteratorProvider() {
+		final var input = IteratorProvider.of(new String[] { "foo", "bar" });
+
+		final var result = (Stream<String>) CollectionUtils.toStream(input);
+
+		assertThat(result).containsExactly("foo", "bar");
+	}
+
+	@Test
+	void throwWhenIteratorNamedMethodDoesNotReturnAnIterator() {
+		var o = IteratorProviderNotUsable.of(new String[] { "Test" });
+		var e = assertThrows(PreconditionViolationException.class, () -> CollectionUtils.toStream(o));
+
+		assertEquals("Method with name 'iterator' does not return java.util.Iterator", e.getMessage());
+	}
+
 	@Test
 	@SuppressWarnings("unchecked")
 	void toStreamWithArray() {
@@ -304,4 +329,29 @@ public Object convert(Object source, ParameterContext context) throws ArgumentCo
 			return source == null ? List.of() : List.of(((String) source).split(","));
 		}
 	}
+
+	/**
+	 * An interface that has a method with name 'iterator', returning a java.util/Iterator as a return type
+	 */
+	private interface IteratorProvider<T> {
+
+		@SuppressWarnings("unused")
+		Iterator<T> iterator();
+
+		static <T> IteratorProvider<T> of(T[] elements) {
+			return () -> Spliterators.iterator(Arrays.spliterator(elements));
+		}
+	}
+
+	/**
+	 * An interface that has a method with name 'iterator', but does not return java.util/Iterator as a return type
+	 */
+	private interface IteratorProviderNotUsable {
+		@SuppressWarnings("unused")
+		Object iterator();
+
+		static <T> IteratorProviderNotUsable of(T[] elements) {
+			return () -> Spliterators.iterator(Arrays.spliterator(elements));
+		}
+	}
 }