Return the correct number of semantic children for the ListView.separated constructor (#48741)

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

@@ -1051,7 +1051,7 @@ class ListView extends BoxScrollView {
            }
            return widget;
          },
-         childCount: _computeSemanticChildCount(itemCount),
+         childCount: _computeActualChildCount(itemCount),
          addAutomaticKeepAlives: addAutomaticKeepAlives,
          addRepaintBoundaries: addRepaintBoundaries,
          addSemanticIndexes: addSemanticIndexes,
@@ -1069,7 +1069,7 @@ class ListView extends BoxScrollView {
          shrinkWrap: shrinkWrap,
          padding: padding,
          cacheExtent: cacheExtent,
-         semanticChildCount: _computeSemanticChildCount(itemCount),
+         semanticChildCount: itemCount,
        );
 
   /// Creates a scrollable, linear array of widgets with a custom child model.
@@ -1215,8 +1215,8 @@ class ListView extends BoxScrollView {
     properties.add(DoubleProperty('itemExtent', itemExtent, defaultValue: null));
   }
 
-  // Helper method to compute the semantic child count for the separated constructor.
+  // Helper method to compute the actual child count for the separated constructor.
-  static int _computeSemanticChildCount(int itemCount) {
+  static int _computeActualChildCount(int itemCount) {
     return math.max(0, itemCount * 2 - 1);
   }
 }

packages/flutter/test/widgets/list_view_builder_test.dart

@@ -304,6 +304,54 @@ void main() {
     expect(find.text('s5'), findsNothing);
     expect(find.text('i6'), findsNothing);
   });
+
+
+  testWidgets('ListView.separated uses correct semanticChildCount', (WidgetTester tester) async {
+    Widget buildFrame({int itemCount}) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: ListView.separated(
+          itemCount: itemCount,
+          itemBuilder: (BuildContext context, int index) {
+            return SizedBox(
+              height: 100.0,
+              child: Text('i$index'),
+            );
+          },
+          separatorBuilder: (BuildContext context, int index) {
+            return SizedBox(
+              height: 10.0,
+              child: Text('s$index'),
+            );
+          },
+        ),
+      );
+    }
+
+    Scrollable scrollable() {
+      return tester.widget<Scrollable>(
+        find.descendant(
+          of: find.byType(ListView),
+          matching: find.byType(Scrollable),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(itemCount: 0));
+    expect(scrollable().semanticChildCount, 0);
+
+    await tester.pumpWidget(buildFrame(itemCount: 1));
+    expect(scrollable().semanticChildCount, 1);
+
+    await tester.pumpWidget(buildFrame(itemCount: 2));
+    expect(scrollable().semanticChildCount, 2);
+
+    await tester.pumpWidget(buildFrame(itemCount: 3));
+    expect(scrollable().semanticChildCount, 3);
+
+    await tester.pumpWidget(buildFrame(itemCount: 4));
+    expect(scrollable().semanticChildCount, 4);
+  });
 }
 
 void check({ List<int> visible = const <int>[], List<int> hidden = const <int>[] }) {