Describe the bug

java.lang.IllegalStateException: Unable to set value of field private final java.util.EnumSet net.minecraft.network.protocol.game.ClientboundPlayerInfoUpdatePacket.a
at com.comphenix.protocol.reflect.accessors.DefaultFieldAccessor.set(DefaultFieldAccessor.java:44) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.reflect.StructureModifier.writeInternal(StructureModifier.java:358) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.reflect.StructureModifier.write(StructureModifier.java:321) ~[ProtocolLib.jar:?]
at net.momirealms.customnameplates.paper.mechanic.nameplate.tag.listener.PlayerInfoListener.onPacketSending(PlayerInfoListener.java:50) ~[CustomNameplates-paper-2.3.0.0-beta-5.jar:?]
at com.comphenix.protocol.injector.SortedPacketListenerList.invokeSendingListener(SortedPacketListenerList.java:219) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.SortedPacketListenerList.invokeUnpackedPacketSending(SortedPacketListenerList.java:204) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.SortedPacketListenerList.invokePacketSending(SortedPacketListenerList.java:149) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.SortedPacketListenerList.invokePacketSending(SortedPacketListenerList.java:139) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.PacketFilterManager.postPacketToListeners(PacketFilterManager.java:553) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.PacketFilterManager.invokePacketSending(PacketFilterManager.java:526) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.netty.manager.NetworkManagerInjector.onPacketSending(NetworkManagerInjector.java:100) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.netty.channel.NettyChannelInjector.processOutbound(NettyChannelInjector.java:553) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.netty.channel.NettyChannelInjector$2.doProxyRunnable(NettyChannelInjector.java:456) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.netty.channel.NettyEventLoopProxy.proxyRunnable(NettyEventLoopProxy.java:43) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.netty.channel.NettyEventLoopProxy.execute(NettyEventLoopProxy.java:252) ~[ProtocolLib.jar:?]
at net.minecraft.network.Connection.sendPacket(Connection.java:432) ~[?:?]
at net.minecraft.network.Connection.send(Connection.java:392) ~[?:?]
at net.minecraft.server.network.ServerCommonPacketListenerImpl.send(ServerCommonPacketListenerImpl.java:255) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.server.network.ServerCommonPacketListenerImpl.send(ServerCommonPacketListenerImpl.java:240) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.server.players.PlayerList.placeNewPlayer(PlayerList.java:369) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.server.network.ServerConfigurationPacketListenerImpl.handleConfigurationFinished(ServerConfigurationPacketListenerImpl.java:130) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.network.protocol.configuration.ServerboundFinishConfigurationPacket.handle(ServerboundFinishConfigurationPacket.java:18) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.network.protocol.configuration.ServerboundFinishConfigurationPacket.a(ServerboundFinishConfigurationPacket.java:9) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.network.protocol.PacketUtils.lambda$ensureRunningOnSameThread$0(PacketUtils.java:54) ~[?:?]
at net.minecraft.server.TickTask.run(TickTask.java:18) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:153) ~[?:?]
at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:24) ~[?:?]
at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:1452) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.server.MinecraftServer.d(MinecraftServer.java:194) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:126) ~[?:?]
at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:1429) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:1352) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.util.thread.BlockableEventLoop.managedBlock(BlockableEventLoop.java:136) ~[?:?]
at net.minecraft.server.MinecraftServer.waitUntilNextTick(MinecraftServer.java:1330) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1220) ~[paper-1.20.4.jar:git-Paper-393]
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:321) ~[paper-1.20.4.jar:git-Paper-393]
at java.lang.Thread.run(Thread.java:833) ~[?:?]
Caused by: java.lang.ClassCastException: Cannot cast java.util.ArrayList to java.util.EnumSet
at java.lang.Class.cast(Class.java:3889) ~[?:?]
at com.comphenix.protocol.reflect.accessors.DefaultFieldAccessor.set(DefaultFieldAccessor.java:41) ~[ProtocolLib.jar:?]
... 36 more

To Reproduce
Steps to reproduce the behavior:

public void onPacketSending(PacketEvent event) {
    PacketContainer packet = event.getPacket();
    List<PlayerInfoData> list = packet.getPlayerInfoDataLists().read(0);
    packet.getPlayerInfoDataLists().write(0, list);
}

Version Info
[03:45:29 INFO]: ProtocolLib version 5.2.0-SNAPSHOT-679
[03:45:29 INFO]: Provides read/write access to the Minecraft protocol.
[03:45:29 INFO]: Authors: dmulloy2 and comphenix

It is happening to me too, when trying to write into the PlayerInfoDataLists

I believe this is a bug, it is trying to write to the actions field instead of the entries field because the actions field happens to be a Collection as well, so the field accessor gets included. Writing to index 1 seems to have worked for me specifically for the player info update packet.

I believe this is a bug, it is trying to write to the actions field instead of the entries field because the actions field happens to be a Collection as well, so the field accessor gets included. Writing to index 1 seems to have worked for me specifically for the player info update packet.

Could you provide a snippet? I am unable to get this working.

I believe this is a bug, it is trying to write to the actions field instead of the entries field because the actions field happens to be a Collection as well, so the field accessor gets included. Writing to index 1 seems to have worked for me specifically for the player info update packet.

Having the same issue here. Snippet would be appreciated

EDIT: This is the solution

public void onPacketSending(PacketEvent event) {
    PacketContainer packet = event.getPacket();
    List<PlayerInfoData> list = packet.getPlayerInfoDataLists().read(1);
    packet.getPlayerInfoDataLists().write(1, list);
}