Fix invalid C-style cast in RenderFrameImplTest.

content/renderer/render_frame_impl_browsertest.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/command_line.h"
 #include "base/debug/leak_annotations.h"
 #include "content/common/frame_messages.h"
 #include "content/common/view_messages.h"
 #include "content/public/test/frame_load_waiter.h"
 #include "content/public/test/render_view_test.h"
@@ skipping 92 matching lines @@
 #else
 #define MAYBE_SubframeWidget SubframeWidget
 #define MAYBE_FrameResize FrameResize
 #define MAYBE_FrameWasShown FrameWasShown
 #endif
 
 // Verify that a frame with a RenderFrameProxy as a parent has its own
 // RenderWidget.
 TEST_F(RenderFrameImplTest, MAYBE_SubframeWidget) {
   EXPECT_TRUE(frame_widget());
-  EXPECT_NE(frame_widget(), (content::RenderWidget*)view_);
+  // We can't convert to RenderWidget* directly, because
+  // it and RenderView are two unrelated base classes
+  // of RenderViewImpl. If a class has multiple base classes,
+  // each base class pointer will be distinct, and direct casts
+  // between unrelated base classes are undefined, even if they share
+  // a common derived class. The compiler has no way in general of
+  // determining the displacement between the two classes, so these
+  // types of casts cannot be implemented in a type safe way.
+  // To overcome this, we make two legal static casts:
+  // first, downcast from RenderView* to RenderViewImpl*,
+  // then upcast from RenderViewImpl* to RenderWidget*.
+  EXPECT_NE(frame_widget(),
+            static_cast<content::RenderWidget*>(
+                static_cast<content::RenderViewImpl*>((view_))));
 }
 
 // Verify a subframe RenderWidget properly processes its viewport being
 // resized.
 TEST_F(RenderFrameImplTest, MAYBE_FrameResize) {
   ViewMsg_Resize_Params resize_params;
   gfx::Size size(200, 200);
   resize_params.screen_info = blink::WebScreenInfo();
   resize_params.new_size = size;
   resize_params.physical_backing_size = size;
@@ skipping 13 matching lines @@
   RenderFrameTestObserver observer(frame());
 
   ViewMsg_WasShown was_shown_message(0, true, ui::LatencyInfo());
   frame_widget()->OnMessageReceived(was_shown_message);
 
   EXPECT_FALSE(frame_widget()->is_hidden());
   EXPECT_TRUE(observer.visible());
 }
 
 }  // namespace