Issue 5258 - Implement Filter::As<>() method to make working with filters easier from C++ code

compiled/filter/CommentFilter.h

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 #pragma once
 
 #include "Filter.h"
 
 class CommentFilter : public Filter
 {
 public:
+  static constexpr Type classType = Type::COMMENT;
   explicit CommentFilter(const String& text);
   static Type Parse(const String& text);
   static CommentFilter* Create(const String& text);
 };
-
-template<>
-inline CommentFilter* Filter::As<CommentFilter>()
-{
-  if ((mType & Type::COMMENT) == Type::COMMENT)

[sergei, 2017/10/12 12:24:34]

Tests would be very helpful, so far it seems it always returns nullptr when the
down-casting is actually possible and does cast when it's impossible.

[Wladimir Palant, 2017/10/13 11:43:28]

This is C++ internals, so it cannot be tested directly. But it will be tested
indirectly by the matcher tests.

Yes, the condition was reversed here. With the code using this not quite done
yet, I didn't have a chance to test this code in action.

-    return nullptr;
-
-  return static_cast<CommentFilter*>(this);
-}