Box/Sliver mismatches should have better error messages (#9525)

We now attempt to explain what went wrong.

Fixes #9509

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

@@ -2668,6 +2668,30 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
 ///
 /// Provides a child model for a render object subclass that has a unique child.
 abstract class RenderObjectWithChildMixin<ChildType extends RenderObject> implements RenderObject {
+  bool debugValidateChild(RenderObject child) {
+    assert(() {
+      if (child is! ChildType) {
+        throw new FlutterError(
+          'A $runtimeType expected a child of type $ChildType but received a '
+          'child of type ${child.runtimeType}.\n'
+          'RenderObjects expect specific types of children because they '
+          'coordinate with their children during layout and paint. For '
+          'example, a RenderSliver cannot be the child of a RenderBox because '
+          'a RenderSliver does not understand the RenderBox layout protocol.\n'
+          '\n'
+          'The $runtimeType that expected a $ChildType child was created by:\n'
+          '  $debugCreator\n'
+          '\n'
+          'The ${child.runtimeType} that did not match the expected child type '
+          'was created by:\n'
+          '  ${child.debugCreator}\n'
+        );
+      }
+      return true;
+    });
+    return true;
+  }
+
   ChildType _child;
   /// The render object's unique child
   ChildType get child => _child;
@@ -2770,6 +2794,30 @@ abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, Parent
   /// The number of children.
   int get childCount => _childCount;
 
+  bool debugValidateChild(RenderObject child) {
+    assert(() {
+      if (child is! ChildType) {
+        throw new FlutterError(
+          'A $runtimeType expected a child of type $ChildType but received a '
+          'child of type ${child.runtimeType}.\n'
+          'RenderObjects expect specific types of children because they '
+          'coordinate with their children during layout and paint. For '
+          'example, a RenderSliver cannot be the child of a RenderBox because '
+          'a RenderSliver does not understand the RenderBox layout protocol.\n'
+          '\n'
+          'The $runtimeType that expected a $ChildType child was created by:\n'
+          '  $debugCreator\n'
+          '\n'
+          'The ${child.runtimeType} that did not match the expected child type '
+          'was created by:\n'
+          '  ${child.debugCreator}\n'
+        );
+      }
+      return true;
+    });
+    return true;
+  }
+
   ChildType _firstChild;
   ChildType _lastChild;
   void _insertIntoChildList(ChildType child, { ChildType after }) {

packages/flutter/lib/src/widgets/binding.dart

@@ -599,6 +599,7 @@ class RenderObjectToWidgetElement<T extends RenderObject> extends RootRenderObje
   @override
   void insertChildRenderObject(RenderObject child, dynamic slot) {
     assert(slot == _rootChildSlot);
+    assert(renderObject.debugValidateChild(child));
     renderObject.child = child;
   }
 

packages/flutter/lib/src/widgets/framework.dart

@@ -4096,6 +4096,7 @@ class SingleChildRenderObjectElement extends RenderObjectElement {
   void insertChildRenderObject(RenderObject child, dynamic slot) {
     final RenderObjectWithChildMixin<RenderObject> renderObject = this.renderObject;
     assert(slot == null);
+    assert(renderObject.debugValidateChild(child));
     renderObject.child = child;
     assert(renderObject == this.renderObject);
   }
@@ -4144,6 +4145,7 @@ class MultiChildRenderObjectElement extends RenderObjectElement {
   @override
   void insertChildRenderObject(RenderObject child, Element slot) {
     final ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>> renderObject = this.renderObject;
+    assert(renderObject.debugValidateChild(child));
     renderObject.insert(child, after: slot?.renderObject);
     assert(renderObject == this.renderObject);
   }

packages/flutter/lib/src/widgets/layout_builder.dart

@@ -131,6 +131,7 @@ class _LayoutBuilderElement extends RenderObjectElement {
   void insertChildRenderObject(RenderObject child, dynamic slot) {
     final RenderObjectWithChildMixin<RenderObject> renderObject = this.renderObject;
     assert(slot == null);
+    assert(renderObject.debugValidateChild(child));
     renderObject.child = child;
     assert(renderObject == this.renderObject);
   }

packages/flutter/lib/src/widgets/overlay.dart

@@ -422,6 +422,7 @@ class _TheatreElement extends RenderObjectElement {
 
   @override
   void insertChildRenderObject(RenderBox child, dynamic slot) {
+    assert(renderObject.debugValidateChild(child));
     if (slot == _onstageSlot) {
       assert(child is RenderStack);
       renderObject.child = child;

packages/flutter/lib/src/widgets/sliver.dart

@@ -647,6 +647,7 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
   void insertChildRenderObject(covariant RenderObject child, int slot) {
     assert(slot != null);
     assert(_currentlyUpdatingChildIndex == slot);
+    assert(renderObject.debugValidateChild(child));
     renderObject.insert(child, after: _currentBeforeChild);
     assert(() {
       final SliverMultiBoxAdaptorParentData childParentData = child.parentData;

packages/flutter/lib/src/widgets/sliver_persistent_header.dart

@@ -125,6 +125,7 @@ class _SliverPersistentHeaderElement extends RenderObjectElement {
 
   @override
   void insertChildRenderObject(covariant RenderObject child, Null slot) {
+    assert(renderObject.debugValidateChild(child));
     renderObject.child = child;
   }
 

packages/flutter/test/widgets/box_sliver_mismatch_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+void main() {
+  testWidgets('Sliver in a box', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new DecoratedBox(
+        decoration: const BoxDecoration(),
+        child: new SliverList(
+          delegate: const SliverChildListDelegate(const <Widget>[]),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isFlutterError);
+
+    await tester.pumpWidget(
+      new Row(
+        children: <Widget>[
+          new SliverList(
+            delegate: const SliverChildListDelegate(const <Widget>[]),
+          ),
+        ],
+      ),
+    );
+
+    expect(tester.takeException(), isFlutterError);
+  });
+
+  testWidgets('Box in a sliver', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new CustomScrollView(
+        slivers: <Widget>[
+          const SizedBox(),
+        ],
+      )
+    );
+
+    expect(tester.takeException(), isFlutterError);
+
+    await tester.pumpWidget(
+      new CustomScrollView(
+        slivers: <Widget>[
+          const SliverPadding(
+            padding: EdgeInsets.zero,
+            sliver: const SizedBox(),
+          ),
+        ],
+      )
+    );
+
+    expect(tester.takeException(), isFlutterError);
+  });
+}