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

compiled/subscription/DownloadableSubscription.h

Index: compiled/subscription/DownloadableSubscription.h
===================================================================
--- a/compiled/subscription/DownloadableSubscription.h
+++ b/compiled/subscription/DownloadableSubscription.h
@@ -20,16 +20,17 @@
 #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);
@@ -49,17 +50,8 @@ public:
       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);
-}