Improve error reporting for RenderObject visitChildren errors (#61455)

* Improve error reporting for RenderObject visitChildren errors

* Nits and fixes

packages/flutter/lib/src/rendering/object.dart

@@ -2243,6 +2243,37 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
       return;
     assert(() {
       if (_needsCompositingBitsUpdate) {
+        if (parent is RenderObject) {
+          final RenderObject parent = this.parent as RenderObject;
+          bool visitedByParent = false;
+          parent.visitChildren((RenderObject child) {
+            if (child == this) {
+              visitedByParent = true;
+            }
+          });
+          if (!visitedByParent) {
+            throw FlutterError.fromParts(<DiagnosticsNode>[
+              ErrorSummary(
+                "A RenderObject was not visited by the parent's visitChildren "
+                'during paint.',
+              ),
+              parent.describeForError(
+                'The parent was',
+              ),
+              describeForError(
+                'The child that was not visited was'
+              ),
+              ErrorDescription(
+                'A RenderObject with children must implement visitChildren and '
+                'call the visitor exactly once for each child; it also should not '
+                'paint children that were removed with dropChild.'
+              ),
+              ErrorHint(
+                'This usually indicates an error in the Flutter framework itself.'
+              ),
+            ]);
+          }
+        }
         throw FlutterError.fromParts(<DiagnosticsNode>[
           ErrorSummary(
             'Tried to paint a RenderObject before its compositing bits were '

packages/flutter/test/widgets/render_object_widget_test.dart

@@ -8,6 +8,8 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
+import '../rendering/recording_canvas.dart';
+
 final BoxDecoration kBoxDecorationA = BoxDecoration(border: nonconst(null));
 final BoxDecoration kBoxDecorationB = BoxDecoration(border: nonconst(null));
 final BoxDecoration kBoxDecorationC = BoxDecoration(border: nonconst(null));
@@ -48,6 +50,31 @@ class TestOrientedBox extends SingleChildRenderObjectWidget {
   }
 }
 
+class TestNonVisitingWidget extends SingleChildRenderObjectWidget {
+  const TestNonVisitingWidget({ Key key, Widget child }) : super(key: key, child: child);
+
+  @override
+  RenderObject createRenderObject(BuildContext context) => TestNonVisitingRenderObject();
+}
+
+class TestNonVisitingRenderObject extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+  @override
+  void performLayout() {
+    child.layout(constraints, parentUsesSize: true);
+    size = child.size;
+  }
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    context.paintChild(child, offset);
+  }
+
+  @override
+  void visitChildren(RenderObjectVisitor visitor) {
+    // oops!
+  }
+}
+
 void main() {
   testWidgets('RenderObjectWidget smoke test', (WidgetTester tester) async {
     await tester.pumpWidget(DecoratedBox(decoration: kBoxDecorationA));
@@ -215,4 +242,25 @@ void main() {
     decoration = renderBox.decoration as BoxDecoration;
     expect(decoration.color, equals(const Color(0xFF0000FF)));
   });
+
+  testWidgets('RenderObject not visiting children provides helpful error message', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      TestNonVisitingWidget(
+        child: Container(color: const Color(0xFFED1D7F)),
+      ),
+    );
+
+    final RenderObject renderObject = tester.renderObject(find.byType(TestNonVisitingWidget));
+    final Canvas testCanvas = TestRecordingCanvas();
+    final PaintingContext testContext = TestRecordingPaintingContext(testCanvas);
+
+    // When a parent fails to visit a child in visitChildren, the child's compositing
+    // bits won't be cleared properly, leading to an exception during paint.
+    renderObject.paint(testContext, Offset.zero);
+
+    final dynamic error = tester.takeException();
+    expect(error, isNotNull, reason: 'RenderObject did not throw when painting');
+    expect(error, isFlutterError);
+    expect(error.toString(), contains("A RenderObject was not visited by the parent's visitChildren"));
+  });
 }