Make warn-unneeded-finalizer warning usable.

tools/clang/blink_gc_plugin/BlinkGCPluginConsumer.cpp

 // 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 "BlinkGCPluginConsumer.h"
 
 #include <algorithm>
 #include <set>
 
 #include "CheckDispatchVisitor.h"
@@ skipping 647 matching lines @@
       NoteField(&field.second, diag_field_requires_finalization_note_);
 }
 
 void BlinkGCPluginConsumer::CheckUnneededFinalization(RecordInfo* info) {
   if (!HasNonEmptyFinalizer(info))
     ReportClassDoesNotRequireFinalization(info);
 }
 
 bool BlinkGCPluginConsumer::HasNonEmptyFinalizer(RecordInfo* info) {
   CXXDestructorDecl* dtor = info->record()->getDestructor();
+
+  // If the destructor is virtual (or one of the bases are by way of the
+  // recursive call below), consider this class as having a non-empty
+  // finalizer. Not doing so runs counter to standard C++ reflexes like
+  //
+  //   class A : public GarbageCollectedMixin {
+  //   public:
+  //     virtual ~A() { };
+  //     virtual void f() = 0;
+  //   };
+  //   class B : public GarbageCollectedFinalized<B>, public A {
+  //     USING_GARBAGE_COLLECTED_MIXIN(B);
+  //   public:
+  //     ~B() override { }
+  //     void f() override { }
+  //   };
+  //
+  // and it is considered a step too far to report a warning for such
+  // explicit usage of empty destructors.
+  if (dtor && dtor->isVirtual())
+      return true;
+
   if (dtor && dtor->isUserProvided()) {
     if (!dtor->hasBody() || !EmptyStmtVisitor::isEmpty(dtor->getBody()))
       return true;
   }
+
+  if (info->GetFinalizeDispatchMethod())
+    return true;
+
   for (auto& base : info->GetBases())
     if (HasNonEmptyFinalizer(base.second.info()))
       return true;
 
   for (auto& field : info->GetFields())
     if (field.second.edge()->NeedsFinalization())
       return true;
 
   return false;
 }
@@ skipping 502 matching lines @@
 void BlinkGCPluginConsumer::NoteField(FieldDecl* field, unsigned note) {
   ReportDiagnostic(field->getLocStart(), note) << field;
 }
 
 void BlinkGCPluginConsumer::NoteOverriddenNonVirtualTrace(
     CXXMethodDecl* overridden) {
   ReportDiagnostic(overridden->getLocStart(),
                    diag_overridden_non_virtual_trace_note_)
       << overridden;
 }