Make Container always clip a decoration if it has a clip edge (#64840)

This allows us to rationalize the nullability of some of our APIs.
Prior to this we'd end up in a situation where Container assumed that
getClipPath would be non-null but Decoration was ok with return null.

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

@@ -64,17 +64,6 @@ class UnderlineTabIndicator extends Decoration {
   _UnderlinePainter createBoxPainter([ VoidCallback onChanged ]) {
     return _UnderlinePainter(this, onChanged);
   }
-}
-
-class _UnderlinePainter extends BoxPainter {
-  _UnderlinePainter(this.decoration, VoidCallback onChanged)
-    : assert(decoration != null),
-      super(onChanged);
-
-  final UnderlineTabIndicator decoration;
-
-  BorderSide get borderSide => decoration.borderSide;
-  EdgeInsetsGeometry get insets => decoration.insets;
 
   Rect _indicatorRectFor(Rect rect, TextDirection textDirection) {
     assert(rect != null);
@@ -88,14 +77,27 @@ class _UnderlinePainter extends BoxPainter {
     );
   }
 
+  @override
+  Path getClipPath(Rect rect, TextDirection textDirection) {
+    return Path()..addRect(_indicatorRectFor(rect, textDirection));
+  }
+}
+
+class _UnderlinePainter extends BoxPainter {
+  _UnderlinePainter(this.decoration, VoidCallback onChanged)
+    : assert(decoration != null),
+      super(onChanged);
+
+  final UnderlineTabIndicator decoration;
+
   @override
   void paint(Canvas canvas, Offset offset, ImageConfiguration configuration) {
     assert(configuration != null);
     assert(configuration.size != null);
     final Rect rect = offset & configuration.size;
     final TextDirection textDirection = configuration.textDirection;
-    final Rect indicator = _indicatorRectFor(rect, textDirection).deflate(borderSide.width / 2.0);
-    final Paint paint = borderSide.toPaint()..strokeCap = StrokeCap.square;
+    final Rect indicator = decoration._indicatorRectFor(rect, textDirection).deflate(decoration.borderSide.width / 2.0);
+    final Paint paint = decoration.borderSide.toPaint()..strokeCap = StrokeCap.square;
     canvas.drawLine(indicator.bottomLeft, indicator.bottomRight, paint);
   }
 }

packages/flutter/lib/src/painting/box_decoration.dart

@@ -215,21 +215,18 @@ class BoxDecoration extends Decoration {
   EdgeInsetsGeometry? get padding => border?.dimensions;
 
   @override
-  Path? getClipPath(Rect rect, TextDirection textDirection) {
-    Path? clipPath;
+  Path getClipPath(Rect rect, TextDirection textDirection) {
     switch (shape) {
       case BoxShape.circle:
         final Offset center = rect.center;
         final double radius = rect.shortestSide / 2.0;
         final Rect square = Rect.fromCircle(center: center, radius: radius);
-        clipPath = Path()..addOval(square);
-        break;
+        return Path()..addOval(square);
       case BoxShape.rectangle:
         if (borderRadius != null)
-          clipPath = Path()..addRRect(borderRadius!.resolve(textDirection).toRRect(rect));
-        break;
+          return Path()..addRRect(borderRadius!.resolve(textDirection).toRRect(rect));
+        return Path()..addRect(rect);
     }
-    return clipPath;
   }
 
   /// Returns a new box decoration that is scaled by the given factor.

packages/flutter/lib/src/painting/decoration.dart

@@ -169,7 +169,18 @@ abstract class Decoration with Diagnosticable {
   BoxPainter createBoxPainter([ VoidCallback onChanged ]);
 
   /// Returns a closed [Path] that describes the outer edge of this decoration.
-  Path? getClipPath(Rect rect, TextDirection textDirection) => null;
+  ///
+  /// The default implementation throws. Subclasses must override this implementation
+  /// to describe the clip path that should be applied to the decoration when it is
+  /// used in a [Container] with an explicit [Clip] behavior.
+  ///
+  /// See also:
+  ///
+  ///  * [Container.clipBehavior], which, if set, uses this method to determine
+  ///    the clip path to use.
+  Path getClipPath(Rect rect, TextDirection textDirection) {
+    throw UnsupportedError('${objectRuntimeType(this, 'This Decoration subclass')} does not expect to be used for clipping.');
+  }
 }
 
 /// A stateful class that can paint a particular [Decoration].

packages/flutter/lib/src/painting/flutter_logo.dart

@@ -173,6 +173,11 @@ class FlutterLogoDecoration extends Decoration {
     return _FlutterLogoPainter(this);
   }
 
+  @override
+  Path getClipPath(Rect rect, TextDirection textDirection) {
+    return Path()..addRect(rect);
+  }
+
   @override
   bool operator ==(Object other) {
     assert(debugAssertIsValid());

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

@@ -1523,9 +1523,9 @@ class RenderClipOval extends _RenderCustomClip<Rect> {
        super(child: child, clipper: clipper, clipBehavior: clipBehavior);
 
   Rect? _cachedRect;
-  Path? _cachedPath;
+  late Path _cachedPath;
 
-  Path? _getClipPath(Rect rect) {
+  Path _getClipPath(Rect rect) {
     if (rect != _cachedRect) {
       _cachedRect = rect;
       _cachedPath = Path()..addOval(_cachedRect!);
@@ -1558,7 +1558,7 @@ class RenderClipOval extends _RenderCustomClip<Rect> {
         needsCompositing,
         offset,
         _clip!,
-        _getClipPath(_clip!)!,
+        _getClipPath(_clip!),
         super.paint,
         clipBehavior: clipBehavior,
         oldLayer: layer as ClipPathLayer,
@@ -1573,7 +1573,7 @@ class RenderClipOval extends _RenderCustomClip<Rect> {
     assert(() {
       if (child != null) {
         super.debugPaintSize(context, offset);
-        context.canvas.drawPath(_getClipPath(_clip!)!.shift(offset), _debugPaint!);
+        context.canvas.drawPath(_getClipPath(_clip!).shift(offset), _debugPaint!);
         _debugText!.paint(context.canvas, offset + Offset((_clip!.width - _debugText!.width) / 2.0, -_debugText!.text!.style!.fontSize! * 1.1));
       }
       return true;

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

@@ -281,6 +281,7 @@ class Container extends StatelessWidget {
        assert(decoration == null || decoration.debugAssertIsValid()),
        assert(constraints == null || constraints.debugAssertIsValid()),
        assert(clipBehavior != null),
+       assert(decoration != null || clipBehavior == Clip.none),
        assert(color == null || decoration == null,
          'Cannot provide both a color and a decoration\n'
          'To provide both, use "decoration: BoxDecoration(color: color)".'
@@ -361,9 +362,14 @@ class Container extends StatelessWidget {
   /// The transformation matrix to apply before painting the container.
   final Matrix4 transform;
 
-  /// The clip behavior when [Container.decoration] has a clipPath.
+  /// The clip behavior when [Container.decoration] is not null.
   ///
-  /// Defaults to [Clip.none].
+  /// Defaults to [Clip.none]. Must be [Clip.none] if [decoration] is null.
+  ///
+  /// If a clip is to be applied, the [Decoration.getClipPath] method
+  /// for the provided decoration must return a clip path. (This is not
+  /// supported by all decorations; the default implementation of that
+  /// method throws an [UnsupportedError].)
   final Clip clipBehavior;
 
   EdgeInsetsGeometry get _paddingIncludingDecoration {
@@ -398,6 +404,7 @@ class Container extends StatelessWidget {
       current = ColoredBox(color: color, child: current);
 
     if (clipBehavior != Clip.none) {
+      assert(decoration != null);
       current = ClipPath(
         clipper: _DecorationClipper(
           textDirection: Directionality.of(context),
@@ -453,7 +460,7 @@ class _DecorationClipper extends CustomClipper<Path> {
   _DecorationClipper({
     TextDirection textDirection,
     @required this.decoration
-  }) : assert (decoration != null),
+  }) : assert(decoration != null),
        textDirection = textDirection ?? TextDirection.ltr;
 
   final TextDirection textDirection;

packages/flutter/test/widgets/container_test.dart

@@ -502,6 +502,36 @@ void main() {
     );
   });
 
+  testWidgets('getClipPath() works for lots of kinds of decorations', (WidgetTester tester) async {
+    Future<void> test(Decoration decoration) async {
+      await tester.pumpWidget(
+        Directionality(
+          textDirection: TextDirection.rtl,
+          child: Center(
+            child: SizedBox(
+              width: 100.0,
+              height: 100.0,
+              child: RepaintBoundary(
+                child: Container(
+                  clipBehavior: Clip.hardEdge,
+                  decoration: decoration,
+                  child: ColoredBox(
+                    color: Colors.yellow.withOpacity(0.5),
+                  ),
+                ),
+              ),
+            ),
+          ),
+        ),
+      );
+      await expectLater(find.byType(Container), matchesGoldenFile('container_test.getClipPath.${decoration.runtimeType}.png'));
+    }
+    await test(const BoxDecoration());
+    await test(const UnderlineTabIndicator());
+    await test(const ShapeDecoration(shape: StadiumBorder()));
+    await test(const FlutterLogoDecoration());
+  });
+
   testWidgets('Container is hittable only when having decorations', (WidgetTester tester) async {
     bool tapped = false;
     await tester.pumpWidget(GestureDetector(