Checkbox borderSide lerp bug fix (#118728)

* theme override fix

* add lerp condition and tests

* remove unwanted changes

* resolve app_bar token version

* update test names

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

@@ -192,10 +192,10 @@ class CheckboxThemeData with Diagnosticable {
 
   // Special case because BorderSide.lerp() doesn't support null arguments
   static BorderSide? _lerpSides(BorderSide? a, BorderSide? b, double t) {
-    if (a == null && b == null) {
+    if (a == null || b == null) {
       return null;
     }
-    return BorderSide.lerp(a!, b!, t);
+    return BorderSide.lerp(a, b, t);
   }
 }
 

packages/flutter/test/material/checkbox_theme_test.dart

@@ -391,6 +391,77 @@ void main() {
     expect(_getCheckboxMaterial(tester), paints..path(color: localThemeFillColor));
     expect(_getCheckboxMaterial(tester), paints..path(color: localThemeFillColor)..path(color: localThemeCheckColor));
   });
+
+  test('CheckboxThemeData lerp with null parameters', () {
+    final CheckboxThemeData lerped = CheckboxThemeData.lerp(null, null, 0.25);
+
+    expect(lerped.mouseCursor, null);
+    expect(lerped.fillColor, null);
+    expect(lerped.checkColor, null);
+    expect(lerped.overlayColor, null);
+    expect(lerped.splashRadius, null);
+    expect(lerped.materialTapTargetSize, null);
+    expect(lerped.visualDensity, null);
+    expect(lerped.shape, null);
+    expect(lerped.side, null);
+  });
+
+  test('CheckboxThemeData lerp from populated to null parameters', () {
+    final CheckboxThemeData theme = CheckboxThemeData(
+      fillColor: MaterialStateProperty.all(const Color(0xfffffff0)),
+      checkColor: MaterialStateProperty.all(const Color(0xfffffff1)),
+      overlayColor: MaterialStateProperty.all(const Color(0xfffffff2)),
+      splashRadius: 3.0,
+      materialTapTargetSize: MaterialTapTargetSize.shrinkWrap,
+      visualDensity: const VisualDensity(vertical: 1.0, horizontal: 1.0),
+      shape: const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(4.0))),
+      side: const BorderSide(width: 4.0),
+    );
+    final CheckboxThemeData lerped = CheckboxThemeData.lerp(theme, null, 0.5);
+
+    expect(lerped.fillColor!.resolve(<MaterialState>{}), const Color(0x80fffff0));
+    expect(lerped.checkColor!.resolve(<MaterialState>{}), const Color(0x80fffff1));
+    expect(lerped.overlayColor!.resolve(<MaterialState>{}), const Color(0x80fffff2));
+    expect(lerped.splashRadius, 1.5);
+    expect(lerped.materialTapTargetSize, null);
+    expect(lerped.visualDensity, null);
+    expect(lerped.shape, const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(2.0))));
+    // Returns null if either lerp value is null.
+    expect(lerped.side, null);
+  });
+
+  test('CheckboxThemeData lerp from populated parameters', () {
+    final CheckboxThemeData themeA = CheckboxThemeData(
+      fillColor: MaterialStateProperty.all(const Color(0xfffffff0)),
+      checkColor: MaterialStateProperty.all(const Color(0xfffffff1)),
+      overlayColor: MaterialStateProperty.all(const Color(0xfffffff2)),
+      splashRadius: 3.0,
+      materialTapTargetSize: MaterialTapTargetSize.shrinkWrap,
+      visualDensity: const VisualDensity(vertical: 1.0, horizontal: 1.0),
+      shape: const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(4.0))),
+      side: const BorderSide(width: 4.0),
+    );
+    final CheckboxThemeData themeB = CheckboxThemeData(
+      fillColor: MaterialStateProperty.all(const Color(0xfffffff3)),
+      checkColor: MaterialStateProperty.all(const Color(0xfffffff4)),
+      overlayColor: MaterialStateProperty.all(const Color(0xfffffff5)),
+      splashRadius: 9.0,
+      materialTapTargetSize: MaterialTapTargetSize.shrinkWrap,
+      visualDensity: const VisualDensity(vertical: 2.0, horizontal: 2.0),
+      shape: const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(1.0))),
+      side: const BorderSide(width: 3.0),
+    );
+    final CheckboxThemeData lerped = CheckboxThemeData.lerp(themeA, themeB, 0.5);
+
+    expect(lerped.fillColor!.resolve(<MaterialState>{}), const Color(0xfffffff1));
+    expect(lerped.checkColor!.resolve(<MaterialState>{}), const Color(0xfffffff2));
+    expect(lerped.overlayColor!.resolve(<MaterialState>{}), const Color(0xfffffff3));
+    expect(lerped.splashRadius, 6);
+    expect(lerped.materialTapTargetSize, MaterialTapTargetSize.shrinkWrap);
+    expect(lerped.visualDensity,  const VisualDensity(vertical: 2.0, horizontal: 2.0));
+    expect(lerped.shape, const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(2.5))));
+    expect(lerped.side, const BorderSide(width: 3.5));
+  });
 }
 
 Future<void> _pointGestureToCheckbox(WidgetTester tester) async {