fix return type for ChildrenProvider

platform/platform-impl/src/com/intellij/ui/tree/AsyncTreeModel.java

@@ -534,7 +534,7 @@ public final class AsyncTreeModel extends AbstractTreeModel implements Identifia
       if (model instanceof ChildrenProvider) {
         //noinspection unchecked
         ChildrenProvider<Object> provider = (ChildrenProvider)model;
-        List<Object> children = provider.getChildren(object);
+        List<?> children = provider.getChildren(object);
         if (children == null) throw new ProcessCanceledException(); // cancel this command
         loaded.children = load(children.size(), index -> children.get(index));
       }

platform/platform-impl/src/com/intellij/ui/tree/BaseTreeModel.java

@@ -20,7 +20,7 @@ public abstract class BaseTreeModel<T> extends AbstractTreeModel implements Chil
 
   @Override
   public int getChildCount(Object object) {
-    List<T> list = getChildren(object);
+    List<? extends T> list = getChildren(object);
     return list == null ? 0 : list.size();
   }
 
@@ -28,14 +28,14 @@ public abstract class BaseTreeModel<T> extends AbstractTreeModel implements Chil
   @Override
   public Object getChild(Object object, int index) {
     if (index < 0) return null;
-    List<T> list = getChildren(object);
+    List<? extends T> list = getChildren(object);
     return list != null && index < list.size() ? list.get(index) : null;
   }
 
   @Override
   @SuppressWarnings("SuspiciousMethodCalls")
   public int getIndexOfChild(Object object, Object child) {
-    List<T> list = getChildren(object);
+    List<? extends T> list = getChildren(object);
     return list == null ? -1 : getChildren(object).indexOf(child);
   }
 }

platform/platform-impl/src/com/intellij/ui/tree/ChildrenProvider.java

@@ -35,5 +35,5 @@ public interface ChildrenProvider<T> {
    * @param parent a tree node
    * @return all children of the specified parent node or {@code null} if they are not ready yet
    */
-  List<T> getChildren(Object parent);
+  List<? extends T> getChildren(Object parent);
 }