[M3] Add error state support for side property of CheckBox (#118386)

* Add error state support for side property

* lint fixes

* lint fixes

Author: esouthren

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

@@ -321,6 +321,7 @@ class Checkbox extends StatefulWidget {
   ///  * [MaterialState.hovered].
   ///  * [MaterialState.focused].
   ///  * [MaterialState.disabled].
+  ///  * [MaterialState.error].
   ///
   /// If this property is not a [MaterialStateBorderSide] and it is
   /// non-null, then it is only rendered when the checkbox's value is
@@ -396,7 +397,8 @@ class _CheckboxState extends State<Checkbox> with TickerProviderStateMixin, Togg
 
   BorderSide? _resolveSide(BorderSide? side) {
     if (side is MaterialStateBorderSide) {
-      return MaterialStateProperty.resolveAs<BorderSide?>(side, states);
+      final Set<MaterialState> sideStates = widget.isError ? (states..add(MaterialState.error)) : states;
+      return MaterialStateProperty.resolveAs<BorderSide?>(side, sideStates);
     }
     if (!states.contains(MaterialState.selected)) {
       return side;

packages/flutter/test/material/checkbox_test.dart

@@ -1656,6 +1656,85 @@ void main() {
     await gestureLongPress.up();
     await tester.pump();
   });
+
+  testWidgets('Checkbox MaterialStateBorderSide applies in error states - M3', (WidgetTester tester) async {
+    final FocusNode focusNode = FocusNode(debugLabel: 'Checkbox');
+    final ThemeData themeData = ThemeData(useMaterial3: true);
+    const Color borderColor = Color(0xffffeb3b);
+    tester.binding.focusManager.highlightStrategy = FocusHighlightStrategy.alwaysTraditional;
+    bool? value = false;
+    Widget buildApp({bool autoFocus = true}) {
+      return MaterialApp(
+        theme: themeData,
+        home: Material(
+          child: Center(
+            child: StatefulBuilder(builder: (BuildContext context, StateSetter setState) {
+              return Checkbox(
+                isError: true,
+                side: MaterialStateBorderSide.resolveWith((Set<MaterialState> states) {
+                  if (states.contains(MaterialState.error)) {
+                    return const BorderSide(color: borderColor, width: 4);
+                  }
+                  return const BorderSide(color: Colors.red, width: 2);
+                }),
+                value: value,
+                onChanged: (bool? newValue) {
+                  setState(() {
+                    value = newValue;
+                  });
+                },
+                autofocus: autoFocus,
+                focusNode: focusNode,
+              );
+            }),
+          ),
+        ),
+      );
+    }
+
+    void expectBorder() {
+      expect(
+        tester.renderObject<RenderBox>(find.byType(Checkbox)),
+        paints
+        ..drrect(
+          color: borderColor,
+          outer: RRect.fromLTRBR(15, 15, 33, 33, const Radius.circular(1)),
+          inner: RRect.fromLTRBR(19, 19, 29, 29, Radius.zero),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildApp());
+    await tester.pumpAndSettle();
+    expectBorder();
+
+    // Focused
+    await tester.pumpWidget(buildApp());
+    await tester.pumpAndSettle();
+    expect(focusNode.hasPrimaryFocus, isTrue);
+    expectBorder();
+
+    // Default color
+    await tester.pumpWidget(Container());
+    await tester.pumpWidget(buildApp(autoFocus: false));
+    await tester.pumpAndSettle();
+    expect(focusNode.hasPrimaryFocus, isFalse);
+    expectBorder();
+
+    // Start hovering
+    final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
+    await gesture.addPointer();
+    await gesture.moveTo(tester.getCenter(find.byType(Checkbox)));
+    await tester.pumpAndSettle();
+    expectBorder();
+
+    // Start pressing
+    final TestGesture gestureLongPress = await tester.startGesture(tester.getCenter(find.byType(Checkbox)));
+    await tester.pump();
+    expectBorder();
+    await gestureLongPress.up();
+    await tester.pump();
+  });
 }
 
 class _SelectedGrabMouseCursor extends MaterialStateMouseCursor {