Adds API in semanticsconfiguration to decide how to merge child semanticsConfigurations (#110730)

* Adds semantics merger API and fix input decorator

* addressing comments

* abstractnode to object

* feature complete

* addressing comments

* fix comments

* conditionally add sort order

* fix bool

* fix test

* more fix

* fix tests

packages/flutter/lib/src/material/input_decorator.dart

@@ -1326,6 +1326,35 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
     return Size.zero;
   }
 
+  ChildSemanticsConfigurationsResult _childSemanticsConfigurationDelegate(List<SemanticsConfiguration> childConfigs) {
+    final ChildSemanticsConfigurationsResultBuilder builder = ChildSemanticsConfigurationsResultBuilder();
+    List<SemanticsConfiguration>? prefixMergeGroup;
+    List<SemanticsConfiguration>? suffixMergeGroup;
+    for (final SemanticsConfiguration childConfig in childConfigs) {
+      if (childConfig.tagsChildrenWith(_InputDecoratorState._kPrefixSemanticsTag)) {
+        prefixMergeGroup ??= <SemanticsConfiguration>[];
+        prefixMergeGroup.add(childConfig);
+      } else if (childConfig.tagsChildrenWith(_InputDecoratorState._kSuffixSemanticsTag)) {
+        suffixMergeGroup ??= <SemanticsConfiguration>[];
+        suffixMergeGroup.add(childConfig);
+      } else {
+        builder.markAsMergeUp(childConfig);
+      }
+    }
+    if (prefixMergeGroup != null) {
+      builder.markAsSiblingMergeGroup(prefixMergeGroup);
+    }
+    if (suffixMergeGroup != null) {
+      builder.markAsSiblingMergeGroup(suffixMergeGroup);
+    }
+    return builder.build();
+  }
+
+  @override
+  void describeSemanticsConfiguration(SemanticsConfiguration config) {
+    config.childConfigurationsDelegate = _childSemanticsConfigurationDelegate;
+  }
+
   @override
   void performLayout() {
     final BoxConstraints constraints = this.constraints;
@@ -1713,12 +1742,16 @@ class _AffixText extends StatelessWidget {
     this.text,
     this.style,
     this.child,
+    this.semanticsSortKey,
+    required this.semanticsTag,
   });
 
   final bool labelIsFloating;
   final String? text;
   final TextStyle? style;
   final Widget? child;
+  final SemanticsSortKey? semanticsSortKey;
+  final SemanticsTag semanticsTag;
 
   @override
   Widget build(BuildContext context) {
@@ -1728,7 +1761,11 @@ class _AffixText extends StatelessWidget {
         duration: _kTransitionDuration,
         curve: _kTransitionCurve,
         opacity: labelIsFloating ? 1.0 : 0.0,
-        child: child ?? (text == null ? null : Text(text!, style: style)),
+        child: Semantics(
+          sortKey: semanticsSortKey,
+          tagForChildren: semanticsTag,
+          child: child ?? (text == null ? null : Text(text!, style: style)),
+        ),
       ),
     );
   }
@@ -1899,6 +1936,11 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
   late AnimationController _floatingLabelController;
   late AnimationController _shakingLabelController;
   final _InputBorderGap _borderGap = _InputBorderGap();
+  static const OrdinalSortKey _kPrefixSemanticsSortOrder = OrdinalSortKey(0);
+  static const OrdinalSortKey _kInputSemanticsSortOrder = OrdinalSortKey(1);
+  static const OrdinalSortKey _kSuffixSemanticsSortOrder = OrdinalSortKey(2);
+  static const SemanticsTag _kPrefixSemanticsTag = SemanticsTag('_InputDecoratorState.prefix');
+  static const SemanticsTag _kSuffixSemanticsTag = SemanticsTag('_InputDecoratorState.suffix');
 
   @override
   void initState() {
@@ -2218,22 +2260,42 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
       ),
     );
 
-    final Widget? prefix = decoration.prefix == null && decoration.prefixText == null ? null :
-      _AffixText(
-        labelIsFloating: widget._labelShouldWithdraw,
-        text: decoration.prefixText,
-        style: MaterialStateProperty.resolveAs(decoration.prefixStyle, materialState) ?? hintStyle,
-        child: decoration.prefix,
-      );
-
-    final Widget? suffix = decoration.suffix == null && decoration.suffixText == null ? null :
-      _AffixText(
-        labelIsFloating: widget._labelShouldWithdraw,
-        text: decoration.suffixText,
-        style: MaterialStateProperty.resolveAs(decoration.suffixStyle, materialState) ?? hintStyle,
-        child: decoration.suffix,
+    final bool hasPrefix = decoration.prefix != null || decoration.prefixText != null;
+    final bool hasSuffix = decoration.suffix != null || decoration.suffixText != null;
+
+    Widget? input = widget.child;
+    // If at least two out of the three are visible, it needs semantics sort
+    // order.
+    final bool needsSemanticsSortOrder = widget._labelShouldWithdraw && (input != null ? (hasPrefix || hasSuffix) : (hasPrefix && hasSuffix));
+
+    final Widget? prefix = hasPrefix
+      ? _AffixText(
+          labelIsFloating: widget._labelShouldWithdraw,
+          text: decoration.prefixText,
+          style: MaterialStateProperty.resolveAs(decoration.prefixStyle, materialState) ?? hintStyle,
+          semanticsSortKey: needsSemanticsSortOrder ? _kPrefixSemanticsSortOrder : null,
+          semanticsTag: _kPrefixSemanticsTag,
+          child: decoration.prefix,
+        )
+      : null;
+
+    final Widget? suffix = hasSuffix
+      ? _AffixText(
+          labelIsFloating: widget._labelShouldWithdraw,
+          text: decoration.suffixText,
+          style: MaterialStateProperty.resolveAs(decoration.suffixStyle, materialState) ?? hintStyle,
+          semanticsSortKey: needsSemanticsSortOrder ? _kSuffixSemanticsSortOrder : null,
+          semanticsTag: _kSuffixSemanticsTag,
+          child: decoration.suffix,
+        )
+      : null;
+
+    if (input != null && needsSemanticsSortOrder) {
+      input = Semantics(
+        sortKey: _kInputSemanticsSortOrder,
+        child: input,
       );
-
+    }
 
     final bool decorationIsDense = decoration.isDense ?? false;
     final double iconSize = decorationIsDense ? 18.0 : 24.0;
@@ -2272,7 +2334,9 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
                 color: _getPrefixIconColor(themeData, defaults),
                 size: iconSize,
               ),
-              child: decoration.prefixIcon!,
+              child: Semantics(
+                child: decoration.prefixIcon,
+              ),
             ),
           ),
         ),
@@ -2297,7 +2361,9 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
                 color: _getSuffixIconColor(themeData, defaults),
                 size: iconSize,
               ),
-              child: decoration.suffixIcon!,
+              child: Semantics(
+                child: decoration.suffixIcon,
+              ),
             ),
           ),
         ),
@@ -2374,7 +2440,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
         isDense: decoration.isDense,
         visualDensity: themeData.visualDensity,
         icon: icon,
-        input: widget.child,
+        input: input,
         label: label,
         hint: hint,
         prefix: prefix,

packages/flutter/lib/src/semantics/semantics.dart

@@ -6,6 +6,7 @@ import 'dart:math' as math;
 import 'dart:ui' as ui;
 import 'dart:ui' show Offset, Rect, SemanticsAction, SemanticsFlag, StringAttribute, TextDirection;
 
+import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/painting.dart' show MatrixUtils, TransformProperty;
 import 'package:flutter/services.dart';
@@ -53,6 +54,20 @@ typedef SemanticsActionHandler = void Function(Object? args);
 /// Used by [SemanticsOwner.onSemanticsUpdate].
 typedef SemanticsUpdateCallback = void Function(ui.SemanticsUpdate update);
 
+/// Signature for the [SemanticsConfiguration.childConfigurationsDelegate].
+///
+/// The input list contains all [SemanticsConfiguration]s that rendering
+/// children want to merge upward. One can tag a render child with a
+/// [SemanticsTag] and look up its [SemanticsConfiguration]s through
+/// [SemanticsConfiguration.tagsChildrenWith].
+///
+/// The return value is the arrangement of these configs, including which
+/// configs continue to merge upward and which configs form sibling merge group.
+///
+/// Use [ChildSemanticsConfigurationsResultBuilder] to generate the return
+/// value.
+typedef ChildSemanticsConfigurationsDelegate = ChildSemanticsConfigurationsResult Function(List<SemanticsConfiguration>);
+
 /// A tag for a [SemanticsNode].
 ///
 /// Tags can be interpreted by the parent of a [SemanticsNode]
@@ -85,6 +100,89 @@ class SemanticsTag {
   String toString() => '${objectRuntimeType(this, 'SemanticsTag')}($name)';
 }
 
+/// The result that contains the arrangement for the child
+/// [SemanticsConfiguration]s.
+///
+/// When the [PipelineOwner] builds the semantics tree, it uses the returned
+/// [ChildSemanticsConfigurationsResult] from
+/// [SemanticsConfiguration.childConfigurationsDelegate] to decide how semantics nodes
+/// should form.
+///
+/// Use [ChildSemanticsConfigurationsResultBuilder] to build the result.
+class ChildSemanticsConfigurationsResult {
+  ChildSemanticsConfigurationsResult._(this.mergeUp, this.siblingMergeGroups);
+
+  /// Returns the [SemanticsConfiguration]s that are supposed to be merged into
+  /// the parent semantics node.
+  ///
+  /// [SemanticsConfiguration]s that are either semantics boundaries or are
+  /// conflicting with other [SemanticsConfiguration]s will form explicit
+  /// semantics nodes. All others will be merged into the parent.
+  final List<SemanticsConfiguration> mergeUp;
+
+  /// The groups of child semantics configurations that want to merge together
+  /// and form a sibling [SemanticsNode].
+  ///
+  /// All the [SemanticsConfiguration]s in a given group that are either
+  /// semantics boundaries or are conflicting with other
+  /// [SemanticsConfiguration]s of the same group will be excluded from the
+  /// sibling merge group and form independent semantics nodes as usual.
+  ///
+  /// The result [SemanticsNode]s from the merges are attached as the sibling
+  /// nodes of the immediate parent semantics node. For example, a `RenderObjectA`
+  /// has a rendering child, `RenderObjectB`. If both of them form their own
+  /// semantics nodes, `SemanticsNodeA` and `SemanticsNodeB`, any semantics node
+  /// created from sibling merge groups of `RenderObjectB` will be attach to
+  /// `SemanticsNodeA` as a sibling of `SemanticsNodeB`.
+  final List<List<SemanticsConfiguration>> siblingMergeGroups;
+}
+
+/// The builder to build a [ChildSemanticsConfigurationsResult] based on its
+/// annotations.
+///
+/// To use this builder, one can use [markAsMergeUp] and
+/// [markAsSiblingMergeGroup] to annotate the arrangement of
+/// [SemanticsConfiguration]s. Once all the configs are annotated, use [build]
+/// to generate the [ChildSemanticsConfigurationsResult].
+class ChildSemanticsConfigurationsResultBuilder {
+  /// Creates a [ChildSemanticsConfigurationsResultBuilder].
+  ChildSemanticsConfigurationsResultBuilder();
+
+  final List<SemanticsConfiguration> _mergeUp = <SemanticsConfiguration>[];
+  final List<List<SemanticsConfiguration>> _siblingMergeGroups = <List<SemanticsConfiguration>>[];
+
+  /// Marks the [SemanticsConfiguration] to be merged into the parent semantics
+  /// node.
+  ///
+  /// The [SemanticsConfiguration] will be added to the
+  /// [ChildSemanticsConfigurationsResult.mergeUp] that this builder builds.
+  void markAsMergeUp(SemanticsConfiguration config) => _mergeUp.add(config);
+
+  /// Marks a group of [SemanticsConfiguration]s to merge together
+  /// and form a sibling [SemanticsNode].
+  ///
+  /// The group of [SemanticsConfiguration]s will be added to the
+  /// [ChildSemanticsConfigurationsResult.siblingMergeGroups] that this builder builds.
+  void markAsSiblingMergeGroup(List<SemanticsConfiguration> configs) => _siblingMergeGroups.add(configs);
+
+  /// Builds a [ChildSemanticsConfigurationsResult] contains the arrangement.
+  ChildSemanticsConfigurationsResult build() {
+    assert((){
+      final Set<SemanticsConfiguration> seenConfigs = <SemanticsConfiguration>{};
+      for (final SemanticsConfiguration config in <SemanticsConfiguration>[..._mergeUp, ..._siblingMergeGroups.flattened]) {
+        assert(
+          seenConfigs.add(config),
+          'Duplicated SemanticsConfigurations. This can happen if the same '
+          'SemanticsConfiguration was marked twice in markAsMergeUp and/or '
+          'markAsSiblingMergeGroup'
+        );
+      }
+      return true;
+    }());
+    return ChildSemanticsConfigurationsResult._(_mergeUp, _siblingMergeGroups);
+  }
+}
+
 /// An identifier of a custom semantics action.
 ///
 /// Custom semantics actions can be provided to make complex user
@@ -3724,6 +3822,25 @@ class SemanticsConfiguration {
     _onDidLoseAccessibilityFocus = value;
   }
 
+  /// A delegate that decides how to handle [SemanticsConfiguration]s produced
+  /// in the widget subtree.
+  ///
+  /// The [SemanticsConfiguration]s are produced by rendering objects in the
+  /// subtree and want to merge up to their parent. This delegate can decide
+  /// which of these should be merged together to form sibling SemanticsNodes and
+  /// which of them should be merged upwards into the parent SemanticsNode.
+  ///
+  /// The input list of [SemanticsConfiguration]s can be empty if the rendering
+  /// object of this semantics configuration is a leaf node.
+  ChildSemanticsConfigurationsDelegate? get childConfigurationsDelegate => _childConfigurationsDelegate;
+  ChildSemanticsConfigurationsDelegate? _childConfigurationsDelegate;
+  set childConfigurationsDelegate(ChildSemanticsConfigurationsDelegate? value) {
+    assert(value != null);
+    _childConfigurationsDelegate = value;
+    // Setting the childConfigsDelegate does not annotate any meaningful
+    // semantics information of the config.
+  }
+
   /// Returns the action handler registered for [action] or null if none was
   /// registered.
   SemanticsActionHandler? getActionHandler(SemanticsAction action) => _actions[action];
@@ -4448,6 +4565,11 @@ class SemanticsConfiguration {
   ///  * [addTagForChildren] to add a tag and for more information about their
   ///    usage.
   Iterable<SemanticsTag>? get tagsForChildren => _tagsForChildren;
+
+  /// Whether this configuration will tag the child semantics nodes with a
+  /// given [SemanticsTag].
+  bool tagsChildrenWith(SemanticsTag tag) => _tagsForChildren?.contains(tag) ?? false;
+
   Set<SemanticsTag>? _tagsForChildren;
 
   /// Specifies a [SemanticsTag] that this configuration wants to apply to all

packages/flutter/test/material/text_field_test.dart

@@ -4375,6 +4375,47 @@ void main() {
     expect(prefixText.style, prefixStyle);
   });
 
+  testWidgets('TextField prefix and suffix create a sibling node', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+    await tester.pumpWidget(
+      overlay(
+        child: TextField(
+          controller: TextEditingController(text: 'some text'),
+          decoration: const InputDecoration(
+            prefixText: 'Prefix',
+            suffixText: 'Suffix',
+          ),
+        ),
+      ),
+    );
+
+    expect(semantics, hasSemantics(TestSemantics.root(
+      children: <TestSemantics>[
+        TestSemantics.rootChild(
+          id: 2,
+          textDirection: TextDirection.ltr,
+          label: 'Prefix',
+        ),
+        TestSemantics.rootChild(
+          id: 1,
+          textDirection: TextDirection.ltr,
+          value: 'some text',
+          actions: <SemanticsAction>[
+            SemanticsAction.tap,
+          ],
+          flags: <SemanticsFlag>[
+            SemanticsFlag.isTextField,
+          ],
+        ),
+        TestSemantics.rootChild(
+          id: 3,
+          textDirection: TextDirection.ltr,
+          label: 'Suffix',
+        ),
+      ],
+    ), ignoreTransform: true, ignoreRect: true));
+  });
+
   testWidgets('TextField with specified suffixStyle', (WidgetTester tester) async {
     final TextStyle suffixStyle = TextStyle(
       color: Colors.pink[500],

packages/flutter/test/widgets/scrollable_test.dart

@@ -1429,6 +1429,51 @@ void main() {
     handle.dispose();
   });
 
+  testWidgets('Two panel semantics is added to the sibling nodes of direct children', (WidgetTester tester) async {
+    final SemanticsHandle handle = tester.ensureSemantics();
+    final UniqueKey key = UniqueKey();
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: ListView(
+          key: key,
+          children: const <Widget>[
+            TextField(
+              autofocus: true,
+              decoration: InputDecoration(
+                prefixText: 'prefix',
+              ),
+            ),
+          ],
+        ),
+      ),
+    ));
+    // Wait for focus.
+    await tester.pumpAndSettle();
+
+    final SemanticsNode scrollableNode = tester.getSemantics(find.byKey(key));
+    SemanticsNode? intermediateNode;
+    scrollableNode.visitChildren((SemanticsNode node) {
+      intermediateNode = node;
+      return true;
+    });
+    SemanticsNode? syntheticScrollableNode;
+    intermediateNode!.visitChildren((SemanticsNode node) {
+      syntheticScrollableNode = node;
+      return true;
+    });
+    expect(syntheticScrollableNode!.hasFlag(ui.SemanticsFlag.hasImplicitScrolling), isTrue);
+
+    int numberOfChild = 0;
+    syntheticScrollableNode!.visitChildren((SemanticsNode node) {
+      expect(node.isTagged(RenderViewport.useTwoPaneSemantics), isTrue);
+      numberOfChild += 1;
+      return true;
+    });
+    expect(numberOfChild, 2);
+
+    handle.dispose();
+  });
+
   testWidgets('Scroll inertia cancel event', (WidgetTester tester) async {
     await pumpTest(tester, null);
     await tester.fling(find.byType(Scrollable), const Offset(0.0, -dragOffset), 1000.0);

packages/flutter/test/widgets/semantics_child_configs_delegate_test.dart

+// Copyright 2014 The Flutter 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/rendering.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'semantics_tester.dart';
+
+void main() {
+  testWidgets('Semantics can merge sibling group', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+    const SemanticsTag first = SemanticsTag('1');
+    const SemanticsTag second = SemanticsTag('2');
+    const SemanticsTag third = SemanticsTag('3');
+    ChildSemanticsConfigurationsResult delegate(List<SemanticsConfiguration> configs) {
+      expect(configs.length, 3);
+      final ChildSemanticsConfigurationsResultBuilder builder = ChildSemanticsConfigurationsResultBuilder();
+      final List<SemanticsConfiguration> sibling = <SemanticsConfiguration>[];
+      // Merge first and third
+      for (final SemanticsConfiguration config in configs) {
+        if (config.tagsChildrenWith(first) || config.tagsChildrenWith(third)) {
+          sibling.add(config);
+        } else {
+          builder.markAsMergeUp(config);
+        }
+      }
+      builder.markAsSiblingMergeGroup(sibling);
+      return builder.build();
+    }
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Semantics(
+          label: 'parent',
+          child: TestConfigDelegate(
+            delegate: delegate,
+            child: Column(
+              children: <Widget>[
+                Semantics(
+                  label: '1',
+                  tagForChildren: first,
+                  child: const SizedBox(width: 100, height: 100),
+                  // this tests that empty nodes disappear
+                ),
+                Semantics(
+                  label: '2',
+                  tagForChildren: second,
+                  child: const SizedBox(width: 100, height: 100),
+                ),
+                Semantics(
+                  label: '3',
+                  tagForChildren: third,
+                  child: const SizedBox(width: 100, height: 100),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(semantics, hasSemantics(TestSemantics.root(
+      children: <TestSemantics>[
+        TestSemantics.rootChild(
+          label: 'parent\n2',
+        ),
+        TestSemantics.rootChild(
+          label: '1\n3',
+        ),
+      ],
+    ), ignoreId: true, ignoreRect: true, ignoreTransform: true));
+  });
+
+  testWidgets('Semantics can drop semantics config', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+    const SemanticsTag first = SemanticsTag('1');
+    const SemanticsTag second = SemanticsTag('2');
+    const SemanticsTag third = SemanticsTag('3');
+    ChildSemanticsConfigurationsResult delegate(List<SemanticsConfiguration> configs) {
+      final ChildSemanticsConfigurationsResultBuilder builder = ChildSemanticsConfigurationsResultBuilder();
+      // Merge first and third
+      for (final SemanticsConfiguration config in configs) {
+        if (config.tagsChildrenWith(first) || config.tagsChildrenWith(third)) {
+          continue;
+        }
+        builder.markAsMergeUp(config);
+      }
+      return builder.build();
+    }
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Semantics(
+          label: 'parent',
+          child: TestConfigDelegate(
+            delegate: delegate,
+            child: Column(
+              children: <Widget>[
+                Semantics(
+                  label: '1',
+                  tagForChildren: first,
+                  child: const SizedBox(width: 100, height: 100),
+                  // this tests that empty nodes disappear
+                ),
+                Semantics(
+                  label: '2',
+                  tagForChildren: second,
+                  child: const SizedBox(width: 100, height: 100),
+                ),
+                Semantics(
+                  label: '3',
+                  tagForChildren: third,
+                  child: const SizedBox(width: 100, height: 100),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(semantics, hasSemantics(TestSemantics.root(
+      children: <TestSemantics>[
+        TestSemantics.rootChild(
+          label: 'parent\n2',
+        ),
+      ],
+    ), ignoreId: true, ignoreRect: true, ignoreTransform: true));
+  });
+
+  testWidgets('Semantics throws when mark the same config twice case 1', (WidgetTester tester) async {
+    const SemanticsTag first = SemanticsTag('1');
+    const SemanticsTag second = SemanticsTag('2');
+    const SemanticsTag third = SemanticsTag('3');
+    ChildSemanticsConfigurationsResult delegate(List<SemanticsConfiguration> configs) {
+      final ChildSemanticsConfigurationsResultBuilder builder = ChildSemanticsConfigurationsResultBuilder();
+      // Marks the same one twice.
+      builder.markAsMergeUp(configs.first);
+      builder.markAsMergeUp(configs.first);
+      return builder.build();
+    }
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Semantics(
+          label: 'parent',
+          child: TestConfigDelegate(
+            delegate: delegate,
+            child: Column(
+              children: <Widget>[
+                Semantics(
+                  label: '1',
+                  tagForChildren: first,
+                  child: const SizedBox(width: 100, height: 100),
+                  // this tests that empty nodes disappear
+                ),
+                Semantics(
+                  label: '2',
+                  tagForChildren: second,
+                  child: const SizedBox(width: 100, height: 100),
+                ),
+                Semantics(
+                  label: '3',
+                  tagForChildren: third,
+                  child: const SizedBox(width: 100, height: 100),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isAssertionError);
+  });
+
+  testWidgets('Semantics throws when mark the same config twice case 2', (WidgetTester tester) async {
+    const SemanticsTag first = SemanticsTag('1');
+    const SemanticsTag second = SemanticsTag('2');
+    const SemanticsTag third = SemanticsTag('3');
+    ChildSemanticsConfigurationsResult delegate(List<SemanticsConfiguration> configs) {
+      final ChildSemanticsConfigurationsResultBuilder builder = ChildSemanticsConfigurationsResultBuilder();
+      // Marks the same one twice.
+      builder.markAsMergeUp(configs.first);
+      builder.markAsSiblingMergeGroup(<SemanticsConfiguration>[configs.first]);
+      return builder.build();
+    }
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Semantics(
+          label: 'parent',
+          child: TestConfigDelegate(
+            delegate: delegate,
+            child: Column(
+              children: <Widget>[
+                Semantics(
+                  label: '1',
+                  tagForChildren: first,
+                  child: const SizedBox(width: 100, height: 100),
+                  // this tests that empty nodes disappear
+                ),
+                Semantics(
+                  label: '2',
+                  tagForChildren: second,
+                  child: const SizedBox(width: 100, height: 100),
+                ),
+                Semantics(
+                  label: '3',
+                  tagForChildren: third,
+                  child: const SizedBox(width: 100, height: 100),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isAssertionError);
+  });
+}
+
+class TestConfigDelegate extends SingleChildRenderObjectWidget {
+  const TestConfigDelegate({super.key, required this.delegate, super.child});
+  final ChildSemanticsConfigurationsDelegate delegate;
+
+  @override
+  RenderTestConfigDelegate createRenderObject(BuildContext context) => RenderTestConfigDelegate(
+    delegate: delegate,
+  );
+
+  @override
+  void updateRenderObject(BuildContext context, RenderTestConfigDelegate renderObject) {
+    renderObject.delegate = delegate;
+  }
+}
+
+class RenderTestConfigDelegate extends RenderProxyBox {
+  RenderTestConfigDelegate({
+    ChildSemanticsConfigurationsDelegate? delegate,
+  }) : _delegate = delegate;
+
+  ChildSemanticsConfigurationsDelegate? get delegate => _delegate;
+  ChildSemanticsConfigurationsDelegate? _delegate;
+  set delegate(ChildSemanticsConfigurationsDelegate? value) {
+    if (value != _delegate) {
+      markNeedsSemanticsUpdate();
+    }
+    _delegate = value;
+  }
+
+  @override
+  void describeSemanticsConfiguration(SemanticsConfiguration config) {
+    config.childConfigurationsDelegate = _delegate;
+  }
+}