feature CtQuery#filterChildren(null)

src/main/java/spoon/reflect/visitor/chain/CtQueryImpl.java

@@ -434,12 +434,14 @@ private void processFilter(CtElement element) {
 			if (element == null) {
 				return;
 			}
-			boolean matches = false;
-			try {
-				matches = filter.matches(element);
-			} catch (ClassCastException e) {
-				onClassCastException(next, e, element);
-				return;
+			boolean matches = true;
+			if (filter != null) {
+				try {
+					matches = filter.matches(element);
+				} catch (ClassCastException e) {
+					onClassCastException(next, e, element);
+					return;
+				}
 			}
 			if (matches) {
 				//send input to output, because Fitler.matches returned true

src/test/java/spoon/test/filters/FilterTest.java

@@ -11,6 +11,7 @@
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Iterator;
 import java.util.List;
 import java.util.TreeSet;
 
@@ -534,6 +535,27 @@ class Context {
 		assertTrue(context.count>0);
 	}
 
+	@Test
+	public void testFilterChildrenWithoutFilterQueryStep() throws Exception {
+		final Launcher launcher = new Launcher();
+		launcher.setArgs(new String[] {"--output-type", "nooutput","--level","info" });
+		launcher.addInputResource("./src/test/java/spoon/test/filters/testclasses");
+		launcher.run();
+
+		//Contract: the filterChildren(null) without Filter returns same results like filter which returns true for each input.
+		List<CtElement> list = launcher.getFactory().Package().getRootPackage().filterChildren(null).list();
+		Iterator<CtElement> iter = list.iterator();
+		launcher.getFactory().Package().getRootPackage().filterChildren(e->{return true;}).forEach(real->{
+			//the elements produced by each query are same
+			CtElement expected = iter.next();
+			if(real!=expected) {
+				assertEquals(expected, real);
+			}
+		});
+		assertTrue(list.size()>0);
+		assertTrue(iter.hasNext()==false);
+	}
+
 	@Test
 	public void testFunctionQueryStep() throws Exception {
 		final Launcher launcher = new Launcher();