Removed support for spliterator.

- Narrowed reflective stream conversion to Iterator providing method with name 'iterator'

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

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

@@ -36,7 +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}.
@@ -140,12 +142,12 @@ 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())//
 				|| Arrays.stream(type.getMethods())//
+						.filter(m -> m.getName().equals("iterator"))//
 						.map(Method::getReturnType)//
-						.anyMatch(returnType -> returnType == Iterator.class || returnType == Spliterator.class));
+						.anyMatch(returnType -> returnType == Iterator.class));
 	}
 
 	/**
@@ -159,10 +161,9 @@ 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>
+	 * <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}
@@ -194,9 +195,6 @@ 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);
 		}
@@ -212,7 +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));
 		}
-		return StreamUtils.tryConvertToStreamByReflection(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

@@ -99,9 +99,7 @@ void toUnmodifiableListThrowsOnMutation() {
 			Collection.class, //
 			Iterable.class, //
 			Iterator.class, //
-			Spliterator.class, //
-			MySpliteratorProvider.class, //
-			MyIteratorProvider.class, //
+			IteratorProvider.class, //
 			Object[].class, //
 			String[].class, //
 			int[].class, //
@@ -123,12 +121,11 @@ 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 }, //
-			MySpliteratorProvider.of(new String[] { "mouse", "bear" }), //
-			MyIteratorProvider.of(new Integer[] { 1, 2, 3, 4, 5 })//
+			IteratorProvider.of(new Integer[] { 1, 2, 3, 4, 5 })//
 		);
 	}
 
@@ -139,6 +136,8 @@ static Stream<Object> objectsConvertibleToStreams() {
 			Object.class, //
 			Integer.class, //
 			String.class, //
+			IteratorProviderNotUsable.class, //
+			Spliterator.class, //
 			int.class, //
 			boolean.class //
 	})
@@ -251,34 +250,22 @@ void toStreamWithIterator() {
 		assertThat(result).containsExactly("foo", "bar");
 	}
 
-	@Test
-	@SuppressWarnings("unchecked")
-	void toStreamWithSpliterator() {
-		final var input = List.of("foo", "bar").spliterator();
-
-		final var result = (Stream<String>) CollectionUtils.toStream(input);
-
-		assertThat(result).containsExactly("foo", "bar");
-	}
-
 	@Test
 	@SuppressWarnings("unchecked")
 	void toStreamWithIteratorProvider() {
-		final var input = MyIteratorProvider.of(new String[] { "foo", "bar" });
+		final var input = IteratorProvider.of(new String[] { "foo", "bar" });
 
 		final var result = (Stream<String>) CollectionUtils.toStream(input);
 
 		assertThat(result).containsExactly("foo", "bar");
 	}
 
 	@Test
-	@SuppressWarnings("unchecked")
-	void toStreamWithSpliteratorProvider() {
-		final var input = MySpliteratorProvider.of(new String[] { "foo", "bar" });
-
-		var result = (Stream<String>) CollectionUtils.toStream(input);
+	void throwWhenIteratorNamedMethodDoesNotReturnAnIterator() {
+		var o = IteratorProviderNotUsable.of(new String[] { "Test" });
+		var e = assertThrows(PreconditionViolationException.class, () -> CollectionUtils.toStream(o));
 
-		assertThat(result).containsExactly("foo", "bar");
+		assertEquals("Method with name 'iterator' does not return java.util.Iterator", e.getMessage());
 	}
 
 	@Test
@@ -345,24 +332,29 @@ public Object convert(Object source, ParameterContext context) throws ArgumentCo
 		}
 	}
 
+	/**
+	 * An interface that has a method with name 'iterator', returning a java.util/Iterator as a return type
+	 */
 	@FunctionalInterface
-	private interface MySpliteratorProvider<T> {
+	private interface IteratorProvider<T> {
 
 		@SuppressWarnings("unused")
-		Spliterator<T> thisReturnsASpliterator();
+		Iterator<T> iterator();
 
-		static <T> MySpliteratorProvider<T> of(T[] elements) {
-			return () -> Arrays.spliterator(elements);
+		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
+	 */
 	@FunctionalInterface
-	private interface MyIteratorProvider<T> {
-
+	private interface IteratorProviderNotUsable {
 		@SuppressWarnings("unused")
-		Iterator<T> thisReturnsAnIterator();
+		Object iterator();
 
-		static <T> MyIteratorProvider<T> of(T[] elements) {
+		static <T> IteratorProviderNotUsable of(T[] elements) {
 			return () -> Spliterators.iterator(Arrays.spliterator(elements));
 		}
 	}