Add Argument serializer for ModIdArgument. EnumArgument can't as vanilla doesn't support extra data.

src/main/java/net/minecraftforge/common/ForgeMod.java

@@ -28,7 +28,9 @@ import net.minecraftforge.fml.event.server.FMLServerStartingEvent;
 import net.minecraftforge.fml.event.server.FMLServerStoppingEvent;
 import net.minecraftforge.fml.javafmlmod.FMLJavaModLoadingContext;
 import net.minecraftforge.server.command.ConfigCommand;
+import net.minecraftforge.server.command.EnumArgument;
 import net.minecraftforge.server.command.ForgeCommand;
+import net.minecraftforge.server.command.ModIdArgument;
 import net.minecraftforge.versions.forge.ForgeVersion;
 import net.minecraftforge.versions.mcp.MCPVersion;
 
@@ -36,6 +38,8 @@ import org.apache.commons.lang3.tuple.Pair;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
+import net.minecraft.command.arguments.ArgumentSerializer;
+import net.minecraft.command.arguments.ArgumentTypes;
 import net.minecraft.data.DataGenerator;
 import net.minecraft.nbt.CompoundNBT;
 import net.minecraft.world.storage.SaveHandler;
@@ -45,12 +49,12 @@ import net.minecraftforge.common.data.ForgeItemTagsProvider;
 import net.minecraftforge.common.data.ForgeRecipeProvider;
 import net.minecraftforge.common.model.animation.CapabilityAnimation;
 import net.minecraftforge.energy.CapabilityEnergy;
+import net.minecraftforge.event.entity.player.PlayerEvent;
 import net.minecraftforge.fml.event.lifecycle.GatherDataEvent;
 import net.minecraftforge.fluids.capability.CapabilityFluidHandler;
 import net.minecraftforge.items.CapabilityItemHandler;
 import net.minecraftforge.fluids.UniversalBucket;
 import net.minecraftforge.fml.common.Mod;
-import net.minecraftforge.event.entity.player.PlayerEvent;
 import org.apache.logging.log4j.Marker;
 import org.apache.logging.log4j.MarkerManager;
 
@@ -118,6 +122,9 @@ public class ForgeMod implements WorldPersistenceHooks.WorldPersistenceHook
         {
             VersionChecker.startVersionCheck();
         }
+
+        //ArgumentTypes.register("forge:enum", EnumArgument.class, new EnumArgument.Serialzier()); //This can't register, it breaks vanilla clients. As the packet serailzier doesn't discard unknown data
+        ArgumentTypes.register("forge:modid", ModIdArgument.class, new ArgumentSerializer<>(ModIdArgument::modIdArgument));
     }
 
     public void serverStarting(FMLServerStartingEvent evt)

src/main/java/net/minecraftforge/server/command/EnumArgument.java

@@ -1,5 +1,6 @@
 package net.minecraftforge.server.command;
 
+import com.google.gson.JsonObject;
 import com.mojang.brigadier.StringReader;
 import com.mojang.brigadier.arguments.ArgumentType;
 import com.mojang.brigadier.context.CommandContext;
@@ -7,13 +8,15 @@ import com.mojang.brigadier.exceptions.CommandSyntaxException;
 import com.mojang.brigadier.suggestion.Suggestions;
 import com.mojang.brigadier.suggestion.SuggestionsBuilder;
 import net.minecraft.command.ISuggestionProvider;
+import net.minecraft.command.arguments.IArgumentSerializer;
+import net.minecraft.network.PacketBuffer;
 
 import java.util.Collection;
 import java.util.concurrent.CompletableFuture;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
-public class EnumArgument<T extends Enum<T>>  implements ArgumentType<T> {
+public class EnumArgument<T extends Enum<T>> implements ArgumentType<T> {
     private final Class<T> enumClass;
 
     public static <R extends Enum<R>> EnumArgument<R> enumArgument(Class<R> enumClass) {
@@ -37,4 +40,27 @@ public class EnumArgument<T extends Enum<T>>  implements ArgumentType<T> {
     public Collection<String> getExamples() {
         return Stream.of(enumClass.getEnumConstants()).map(Object::toString).collect(Collectors.toList());
     }
+
+    @SuppressWarnings({"rawtypes", "unchecked"})
+    public static class Serialzier implements IArgumentSerializer<EnumArgument> {
+        @Override
+        public void write(EnumArgument argument, PacketBuffer buffer) {
+            buffer.writeString(argument.enumClass.getName());
+        }
+
+        @Override
+        public EnumArgument<?> read(PacketBuffer buffer) {
+            try {
+                String name = buffer.readString();
+                return new EnumArgument(Class.forName(name));
+            } catch (ClassNotFoundException e) {
+                return null;
+            }
+        }
+
+        @Override
+        public void write(EnumArgument argument, JsonObject json) {
+            json.addProperty("enum", argument.enumClass.getName());
+        }
+    }
 }

src/main/java/net/minecraftforge/server/command/ModIdArgument.java

@@ -1,5 +1,6 @@
 package net.minecraftforge.server.command;
 
+import com.google.gson.JsonObject;
 import com.mojang.brigadier.StringReader;
 import com.mojang.brigadier.arguments.ArgumentType;
 import com.mojang.brigadier.context.CommandContext;
@@ -7,6 +8,8 @@ import com.mojang.brigadier.exceptions.CommandSyntaxException;
 import com.mojang.brigadier.suggestion.Suggestions;
 import com.mojang.brigadier.suggestion.SuggestionsBuilder;
 import net.minecraft.command.ISuggestionProvider;
+import net.minecraft.command.arguments.IArgumentSerializer;
+import net.minecraft.network.PacketBuffer;
 import net.minecraftforge.fml.ModContainer;
 import net.minecraftforge.fml.ModList;
 
@@ -36,4 +39,22 @@ public class ModIdArgument implements ArgumentType<String> {
     public Collection<String> getExamples() {
         return EXAMPLES;
     }
+
+    public static class Serialzier implements IArgumentSerializer<ModIdArgument> {
+
+        @Override
+        public void write(ModIdArgument argument, PacketBuffer buffer) {
+        }
+
+        @Override
+        public ModIdArgument read(PacketBuffer buffer) {
+            return null;
+        }
+
+        @Override
+        public void write(ModIdArgument argument, JsonObject json) {
+
+        }
+
+    }
 }