Restore ability to modify selection in GroupRequest within DeleteAction

The list returned by getSelectedEditParts() should be a view of the
internal selection, similar to getSelectedObjects(). Meaning callers are
able to modify by the selection by calling set(int,Object).

In order to reduce the chance of an undesirable ClassCastException, we
only check whether the first element in the list is an EditPart,
assuming that all remaining elements are as well. I.e. we don't expect a
"mixed" selection of different types. This is similar to what has
already been done in e.g. the DeleteAction or the MatchSizeAction.

This is a follow-up to 78c3fed.

Resolves #616

org.eclipse.gef.tests/src/org/eclipse/gef/test/GraphicalViewerTest.java

@@ -13,14 +13,27 @@
 
 package org.eclipse.gef.test;
 
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 
+import java.util.List;
+
 import org.eclipse.jface.viewers.IStructuredSelection;
 import org.eclipse.jface.viewers.StructuredSelection;
+import org.eclipse.ui.IWorkbenchPart;
 import org.eclipse.ui.PlatformUI;
 
+import org.eclipse.draw2d.Figure;
+import org.eclipse.draw2d.IFigure;
+
+import org.eclipse.gef.EditPart;
 import org.eclipse.gef.GraphicalViewer;
+import org.eclipse.gef.Request;
+import org.eclipse.gef.commands.Command;
+import org.eclipse.gef.editparts.AbstractGraphicalEditPart;
+import org.eclipse.gef.requests.GroupRequest;
+import org.eclipse.gef.ui.actions.DeleteAction;
 import org.eclipse.gef.ui.parts.GraphicalViewerImpl;
 
 import org.junit.Before;
@@ -54,4 +67,42 @@ public void testSetNullSelection() {
 		assertThrows(NullPointerException.class, () -> viewer.setSelection(selection));
 		assertTrue(viewer.getSelectedEditParts().isEmpty());
 	}
+
+	@Test
+	public void testDeleteSelection() {
+		IStructuredSelection selection = new StructuredSelection(new DummyEditPart());
+		viewer.setSelection(selection);
+
+		DeleteAction deleteAction = new DeleteAction((IWorkbenchPart) null);
+		deleteAction.setSelectionProvider(viewer);
+		deleteAction.update();
+		deleteAction.run();
+	}
+
+	private static class DummyEditPart extends AbstractGraphicalEditPart {
+		@Override
+		protected IFigure createFigure() {
+			return new Figure();
+		}
+
+		@Override
+		protected void createEditPolicies() {
+			// nothing to do
+		}
+
+		@Override
+		public Command getCommand(Request request) {
+			assertEquals(request.getType(), REQ_DELETE);
+			if (REQ_DELETE.equals(request.getType())) {
+				GroupRequest groupRequest = (GroupRequest) request;
+				DummyEditPart newEditPart = new DummyEditPart();
+
+				@SuppressWarnings("unchecked")
+				List<EditPart> editParts = (List<EditPart>) groupRequest.getEditParts();
+				editParts.set(0, newEditPart);
+				assertEquals(groupRequest.getEditParts(), List.of(newEditPart));
+			}
+			return super.getCommand(request);
+		}
+	}
 }

org.eclipse.gef/src/org/eclipse/gef/ui/actions/PasteTemplateAction.java

@@ -66,7 +66,7 @@ protected boolean calculateEnabled() {
 	 */
 	protected Command createPasteCommand() {
 		Command result = null;
-		List<Object> selection = getSelectedObjects();
+		List<?> selection = getSelectedObjects();
 		if (selection != null && selection.size() == 1 && selection.get(0) instanceof GraphicalEditPart gep) {
 			Object template = getClipboardContents();
 			if (template != null) {

org.eclipse.gef/src/org/eclipse/gef/ui/actions/SelectionAction.java

@@ -65,26 +65,41 @@ public void dispose() {
 	}
 
 	/**
-	 * Returns a <code>List</code> containing the currently selected objects.
+	 * Returns a {@code List<EditPart} containing the currently selected objects.
+	 * The {@code set} method is supported but the {@code add}, {@code addAll},
+	 * {@code remove} {@code removeAll} methods are not. Changes to this list modify
+	 * the internal {@link #selection}.
 	 *
 	 * @return A List containing the currently selected objects.
 	 */
-	protected List<Object> getSelectedObjects() {
+	protected List<?> getSelectedObjects() {
 		if (!(getSelection() instanceof IStructuredSelection)) {
 			return Collections.emptyList();
 		}
 		return ((IStructuredSelection) getSelection()).toList();
 	}
 
 	/**
-	 * Returns a <code>List<EditPart></code> containing the currently selected
-	 * EditParts.
+	 * Returns a {@code List<EditPart} containing the currently selected EditParts.
+	 * If elements not of type EditPart are selected, an empty list is returned. The
+	 * {@code set} method is supported but the {@code add}, {@code addAll},
+	 * {@code remove} {@code removeAll} methods are not. Changes to this list modify
+	 * the internal {@link #selection}.<br>
+	 * <em>Note:</em> This method only checks whether the first selected element is
+	 * of type {@link EditPart} and then performs a lazy cast from {@code List<?>}
+	 * to {@code List<EditPart>}. It should therefore only be called when a single
+	 * type of objects are selected.
 	 *
 	 * @return A List containing the currently selected EditParts.
 	 * @since 3.20
 	 */
+	@SuppressWarnings("unchecked") // We don't expect a "mixed" selection
 	protected final List<EditPart> getSelectedEditParts() {
-		return getSelectedObjects().stream().filter(EditPart.class::isInstance).map(EditPart.class::cast).toList();
+		List<?> selectedObjects = getSelectedObjects();
+		if (selectedObjects.isEmpty() || !(selectedObjects.get(0) instanceof EditPart)) {
+			return Collections.emptyList();
+		}
+		return (List<EditPart>) selectedObjects;
 	}
 
 	/**