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

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

Author: Hans Zuidervaart

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

@@ -0,0 +1,76 @@
+/*
+ * 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("$it should be even") { assertTrue(it % 2 == 0) } }
+            .take(10)
+
+        @TestFactory
+        fun `Is anagram tests`(): Sequence<DynamicTest> {
+            infix fun CharSequence.isAngramOf(other: CharSequence) = groupBy { it } == other.groupBy { it }
+
+            infix fun CharSequence.`should be an anagram of`(other: CharSequence) =
+                dynamicTest("'$this' should be an anagram of '$other'") { assertTrue(this isAngramOf other) }
+
+            infix fun CharSequence.`should not be an anagram of`(other: CharSequence) =
+                dynamicTest("'$this' should not be an anagram of '$other'") { assertFalse(this isAngramOf other) }
+
+            return sequenceOf(
+                "a gentleman" `should be an anagram of` "elegant man",
+                "laptop machines" `should be an anagram of` "apple macintosh",
+                "salvador dali" `should be an anagram of` "avida dollars",
+                "a gentleman" `should not be an anagram of` "spider man",
+                "laptop computers" `should not be an anagram of` "apple macintosh",
+                "salvador dali" `should not be an anagram of` "picasso"
+            )
+        }
+
+        @TestFactory
+        fun `Consecutive fibonacci nr ratios, should converge to golden ratio as n increases`(): Sequence<DynamicTest> {
+            val scale = 5
+            val goldenRatio = (ONE + 5.toBigDecimal().sqrt(MathContext(scale + 10, Rounding.HALF_UP)))
+                .divide(2.toBigDecimal(), scale, Rounding.HALF_UP)
+
+            fun shouldApproximateGoldenRatio(cur: BigDecimal, next: BigDecimal) =
+                next.divide(cur, scale, Rounding.HALF_UP).let {
+                    dynamicTest("$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.MethodSource
+import java.time.Month
+import java.util.*
+
+/**
+ * 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(
+        arrayOf(1, Month.JANUARY),
+        arrayOf(3, Month.MARCH),
+        arrayOf(8, Month.AUGUST),
+        arrayOf(5, Month.MAY),
+        arrayOf(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;
@@ -99,7 +101,7 @@ 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);
@@ -138,8 +140,12 @@ public static boolean isConvertibleToStream(Class<?> type) {
 				|| LongStream.class.isAssignableFrom(type)//
 				|| Iterable.class.isAssignableFrom(type)//
 				|| Iterator.class.isAssignableFrom(type)//
+				|| Spliterator.class.isAssignableFrom(type)//
 				|| Object[].class.isAssignableFrom(type)//
-				|| (type.isArray() && type.getComponentType().isPrimitive()));
+				|| (type.isArray() && type.getComponentType().isPrimitive())//
+				|| Arrays.stream(type.getMethods())//
+						.map(Method::getReturnType)//
+						.anyMatch(returnType -> returnType == Iterator.class || returnType == Spliterator.class));
 	}
 
 	/**
@@ -153,8 +159,10 @@ public static boolean isConvertibleToStream(Class<?> type) {
 	 * <li>{@link Collection}</li>
 	 * <li>{@link Iterable}</li>
 	 * <li>{@link Iterator}</li>
+	 * <li>{@link Spliterator}</li>
 	 * <li>{@link Object} array</li>
 	 * <li>primitive array</li>
+	 * <li>An object that contains an iterator or spliterator returning method</li>
 	 * </ul>
 	 *
 	 * @param object the object to convert into a stream; never {@code null}
@@ -186,6 +194,9 @@ public static Stream<?> toStream(Object object) {
 		if (object instanceof Iterator) {
 			return stream(spliteratorUnknownSize((Iterator<?>) object, ORDERED), false);
 		}
+		if (object instanceof Spliterator) {
+			return stream((Spliterator<?>) object, false);
+		}
 		if (object instanceof Object[]) {
 			return Arrays.stream((Object[]) object);
 		}
@@ -201,8 +212,7 @@ 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 StreamUtils.tryConvertToStreamByReflection(object);
 	}
 
 	/**

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

@@ -0,0 +1,109 @@
+/*
+ * 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.platform.commons.util;
+
+import static java.util.Spliterator.ORDERED;
+import static java.util.Spliterators.spliteratorUnknownSize;
+import static java.util.stream.StreamSupport.stream;
+
+import java.lang.reflect.Method;
+import java.util.Arrays;
+import java.util.Iterator;
+import java.util.Optional;
+import java.util.Spliterator;
+import java.util.stream.Stream;
+
+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 Stream Streams}.
+ *
+ * @since 5.10
+ */
+final class StreamUtils {
+
+	private StreamUtils() {
+	}
+
+	static Stream<?> tryConvertToStreamByReflection(Object object) {
+		Preconditions.notNull(object, "Object must not be null");
+		Class<?> theClass = object.getClass();
+		try {
+			String name = "iterator";
+			Method method = theClass.getMethod(name);
+			if (method.getReturnType() == Iterator.class) {
+				return stream(() -> tryIteratorToSpliterator(object, method), ORDERED, false);
+			}
+			else {
+				throw new IllegalStateException(
+					"Method with name 'iterator' does not return " + Iterator.class.getName());
+			}
+		}
+		catch (NoSuchMethodException | IllegalStateException e) {
+			return tryConvertToStreamBySpliterator(object, e);
+		}
+	}
+
+	private static Stream<?> tryConvertToStreamBySpliterator(Object object, Exception e) {
+		try {
+			String name = "spliterator";
+			Method method = object.getClass().getMethod(name);
+			if (method.getReturnType() == Spliterator.class) {
+				return stream(() -> tryInvokeSpliterator(object, method), ORDERED, false);
+			}
+			else {
+				throw new IllegalStateException(
+					"Method with name '" + name + "' does not return " + Spliterator.class.getName());
+			}
+		}
+		catch (NoSuchMethodException | IllegalStateException ex) {
+			ex.addSuppressed(e);
+			return tryConvertByIteratorSpliteratorReturnType(object, ex);
+		}
+	}
+
+	private static Stream<?> tryConvertByIteratorSpliteratorReturnType(Object object, Exception ex) {
+		return streamFromSpliteratorSupplier(object)//
+				.orElseGet(() -> streamFromIteratorSupplier(object)//
+						.orElseThrow(() -> //
+						new PreconditionViolationException(//
+							"Cannot convert instance of " + object.getClass().getName() + " into a Stream: " + object,
+							ex)));
+	}
+
+	private static Optional<Stream<?>> streamFromSpliteratorSupplier(Object object) {
+		return Arrays.stream(object.getClass().getMethods())//
+				.filter(m -> m.getReturnType() == Spliterator.class)//
+				.findFirst()//
+				.map(m -> stream(() -> tryInvokeSpliterator(object, m), ORDERED, false));//
+	}
+
+	private static Optional<Stream<?>> streamFromIteratorSupplier(Object object) {
+		return Arrays.stream(object.getClass().getMethods())//
+				.filter(m -> m.getReturnType() == Iterator.class)//
+				.findFirst()//
+				.map(m -> stream(() -> tryIteratorToSpliterator(object, m), ORDERED, false));//
+	}
+
+	private static Spliterator<?> tryInvokeSpliterator(Object object, Method method) {
+		return Try.call(() -> (Spliterator<?>) method.invoke(object))//
+				.getOrThrow(e -> new JUnitException("Cannot invoke method " + method.getName() + " onto " + object, e));//
+	}
+
+	private static Spliterator<?> tryIteratorToSpliterator(Object object, Method method) {
+		return Try.call(() -> method.invoke(object))//
+				.andThen(m -> Try.call(() -> spliteratorUnknownSize((Iterator<?>) m, ORDERED)))//
+				.getOrThrow(e -> new JUnitException("Cannot invoke method " + method.getName() + " onto " + object, e));//
+	}
+
+}