fix(Core/Arena): allow crossfaction arena teams (#16823)

src/server/game/Battlegrounds/Arena.cpp

@@ -17,6 +17,7 @@
 
 #include "Arena.h"
 #include "ArenaTeamMgr.h"
+#include "GroupMgr.h"
 #include "Log.h"
 #include "ObjectAccessor.h"
 #include "Player.h"
@@ -93,6 +94,47 @@ void Arena::AddPlayer(Player* player)
     }
 
     UpdateArenaWorldState();
+
+    Group* group = player->GetGroup();
+    if (group)
+    {
+        // Hackfix for crossfaction arenas, recreate group when joining
+        // Without this, players in a crossfaction arena group would not be able to cast beneficial spells on their teammates
+
+        std::vector<Player*> members;
+        bool isCrossfaction = false;
+        for (Group::member_citerator mitr = group->GetMemberSlots().begin(); mitr != group->GetMemberSlots().end(); ++mitr)
+        {
+            Player* member = ObjectAccessor::FindPlayer(mitr->guid);
+            if (!member || member->GetGUID() == player->GetGUID())
+            {
+                continue;
+            }
+            members.push_back(member);
+            if (member->GetTeamId(true) != player->GetTeamId(true))
+            {
+                isCrossfaction = true;
+            }
+        }
+
+        if (isCrossfaction)
+        {
+            for (Player* member : members)
+            {
+                member->RemoveFromGroup();
+            }
+            group->Disband();
+
+            group = new Group();
+            SetBgRaid(player->GetBgTeamId(), group);
+            group->Create(player);
+            sGroupMgr->AddGroup(group);
+            for (Player* member : members)
+            {
+                group->AddMember(member);
+            }
+        }
+    }
 }
 
 void Arena::RemovePlayer(Player* /*player*/)