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

compiled/subscription/DownloadableSubscription.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 <cstdint>
 
 #include "Subscription.h"
 #include "../bindings/runtime.h"
 
 class DownloadableSubscription : public Subscription
 {
 public:
+  static constexpr Type classType = Type::DOWNLOADABLE;
   explicit DownloadableSubscription(const String& id);
 
   SUBSCRIPTION_PROPERTY(bool, mFixedTitle, SUBSCRIPTION_FIXEDTITLE,
       GetFixedTitle, SetFixedTitle);
   SUBSCRIPTION_STRING_PROPERTY(mHomepage, SUBSCRIPTION_HOMEPAGE,
       GetHomepage, SetHomepage);
   SUBSCRIPTION_PROPERTY(uint64_t, mLastCheck, SUBSCRIPTION_LASTCHECK,
       GetLastCheck, SetLastCheck);
   SUBSCRIPTION_PROPERTY(uint64_t, mHardExpiration, NONE,
       GetHardExpiration, SetHardExpiration);
   SUBSCRIPTION_PROPERTY(uint64_t, mSoftExpiration, NONE,
       GetSoftExpiration, SetSoftExpiration);
   SUBSCRIPTION_PROPERTY(uint64_t, mLastDownload, SUBSCRIPTION_LASTDOWNLOAD,
       GetLastDownload, SetLastDownload);
   SUBSCRIPTION_STRING_PROPERTY(mDownloadStatus, SUBSCRIPTION_DOWNLOADSTATUS,
       GetDownloadStatus, SetDownloadStatus);
   SUBSCRIPTION_PROPERTY(uint64_t, mLastSuccess, NONE,
       GetLastSuccess, SetLastSuccess);
   SUBSCRIPTION_PROPERTY(int, mErrorCount, SUBSCRIPTION_ERRORS,
       GetErrorCount, SetErrorCount);
   SUBSCRIPTION_PROPERTY(uint64_t, mDataRevision, NONE,
       GetDataRevision, SetDataRevision);
   SUBSCRIPTION_STRING_PROPERTY(mRequiredVersion, NONE,
       GetRequiredVersion, SetRequiredVersion);
   SUBSCRIPTION_PROPERTY(int, mDownloadCount, NONE,
       GetDownloadCount, SetDownloadCount);
 
   OwnedString BINDINGS_EXPORTED Serialize() const;
 };
-
-template<>
-inline DownloadableSubscription* Subscription::As<DownloadableSubscription>()
-{
-  if (mType != Type::DOWNLOADABLE)
-    return nullptr;
-
-  return static_cast<DownloadableSubscription*>(this);
-}