Fix tag related issues when connecting to a vanilla server.

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

@@ -19,11 +19,18 @@
 
 package net.minecraftforge.common;
 
+import java.util.Arrays;
+import java.util.stream.Collectors;
+
 import net.minecraft.block.Block;
 import net.minecraft.item.Item;
 import net.minecraft.tags.BlockTags;
 import net.minecraft.tags.ITag;
 import net.minecraft.tags.ItemTags;
+import net.minecraft.util.RegistryKey;
+import net.minecraft.util.registry.Registry;
+import net.minecraftforge.common.util.ForgeNetworkTagManager;
+import net.minecraftforge.registries.IForgeRegistryEntry;
 
 public class Tags
 {
@@ -37,10 +44,10 @@ public class Tags
         public static final ITag.INamedTag<Block> DIRT = tag("dirt");
         public static final ITag.INamedTag<Block> END_STONES = tag("end_stones");
         public static final ITag.INamedTag<Block> FENCE_GATES = tag("fence_gates");
-        public static final ITag.INamedTag<Block> FENCE_GATES_WOODEN = tag("fence_gates/wooden");
+        public static final ITag.INamedTag<Block> FENCE_GATES_WOODEN = defaulted("fence_gates/wooden", net.minecraft.block.Blocks.OAK_FENCE_GATE, net.minecraft.block.Blocks.BIRCH_FENCE_GATE, net.minecraft.block.Blocks.SPRUCE_FENCE_GATE, net.minecraft.block.Blocks.JUNGLE_FENCE_GATE, net.minecraft.block.Blocks.DARK_OAK_FENCE_GATE, net.minecraft.block.Blocks.ACACIA_FENCE_GATE);
         public static final ITag.INamedTag<Block> FENCES = tag("fences");
         public static final ITag.INamedTag<Block> FENCES_NETHER_BRICK = tag("fences/nether_brick");
-        public static final ITag.INamedTag<Block> FENCES_WOODEN = tag("fences/wooden");
+        public static final ITag.INamedTag<Block> FENCES_WOODEN = defaulted("fences/wooden", net.minecraft.block.Blocks.OAK_FENCE, net.minecraft.block.Blocks.BIRCH_FENCE, net.minecraft.block.Blocks.SPRUCE_FENCE, net.minecraft.block.Blocks.JUNGLE_FENCE, net.minecraft.block.Blocks.DARK_OAK_FENCE, net.minecraft.block.Blocks.ACACIA_FENCE);
 
         public static final ITag.INamedTag<Block> GLASS = tag("glass");
         public static final ITag.INamedTag<Block> GLASS_BLACK = tag("glass/black");
@@ -115,6 +122,12 @@ public class Tags
         {
             return BlockTags.makeWrapperTag("forge:" + name);
         }
+
+        @SuppressWarnings("deprecation")
+        private static ITag.INamedTag<Block> defaulted(String name, Block... values)
+        {
+            return ForgeNetworkTagManager.defaulted(Registry.field_239711_l_, tag(name), Arrays.asList(values).stream().map(e -> e.getRegistryName()).collect(Collectors.toSet()));
+        }
     }
 
     public static class Items
@@ -162,10 +175,10 @@ public class Tags
         public static final ITag.INamedTag<Item> ENDER_PEARLS = tag("ender_pearls");
         public static final ITag.INamedTag<Item> FEATHERS = tag("feathers");
         public static final ITag.INamedTag<Item> FENCE_GATES = tag("fence_gates");
-        public static final ITag.INamedTag<Item> FENCE_GATES_WOODEN = tag("fence_gates/wooden");
+        public static final ITag.INamedTag<Item> FENCE_GATES_WOODEN = defaulted("fence_gates/wooden", Blocks.FENCE_GATES_WOODEN);
         public static final ITag.INamedTag<Item> FENCES = tag("fences");
         public static final ITag.INamedTag<Item> FENCES_NETHER_BRICK = tag("fences/nether_brick");
-        public static final ITag.INamedTag<Item> FENCES_WOODEN = tag("fences/wooden");
+        public static final ITag.INamedTag<Item> FENCES_WOODEN = defaulted("fences/wooden", Blocks.FENCES_WOODEN);
         public static final ITag.INamedTag<Item> GEMS = tag("gems");
         public static final ITag.INamedTag<Item> GEMS_DIAMOND = tag("gems/diamond");
         public static final ITag.INamedTag<Item> GEMS_EMERALD = tag("gems/emerald");
@@ -271,5 +284,11 @@ public class Tags
         {
             return ItemTags.makeWrapperTag("forge:" + name);
         }
+
+        @SuppressWarnings("deprecation")
+        private static ITag.INamedTag<Item> defaulted(String name, ITag.INamedTag<Block> parent)
+        {
+            return ForgeNetworkTagManager.defaulted(Registry.field_239714_o_, tag(name), RegistryKey.func_240903_a_(Registry.field_239711_l_, parent.func_230234_a_()));
+        }
     }
 }

src/main/java/net/minecraftforge/common/data/ForgeBlockTagsProvider.java

@@ -22,7 +22,6 @@ package net.minecraftforge.common.data;
 import net.minecraft.data.DataGenerator;
 import net.minecraft.item.DyeColor;
 import net.minecraft.tags.ITag;
-import net.minecraft.tags.Tag;
 import net.minecraft.util.ResourceLocation;
 import net.minecraftforge.common.Tags;
 import net.minecraftforge.registries.ForgeRegistries;
@@ -47,6 +46,7 @@ public class ForgeBlockTagsProvider extends BlockTagsProvider
         super(gen);
     }
 
+    @SuppressWarnings("unchecked")
     @Override
     public void registerTags()
     {

src/main/java/net/minecraftforge/common/data/ForgeItemTagsProvider.java

@@ -47,6 +47,7 @@ public class ForgeItemTagsProvider extends ItemTagsProvider
         super(gen, blockTagProvider);
     }
 
+    @SuppressWarnings("unchecked")
     @Override
     public void registerTags()
     {

src/main/java/net/minecraftforge/common/extensions/IForgeTagBuilder.java

@@ -35,6 +35,7 @@ public interface IForgeTagBuilder<T>
         return (TagsProvider.Builder<T>) this;
     }
 
+    @SuppressWarnings("unchecked")
     default TagsProvider.Builder<T> addTags(ITag.INamedTag<T>... values) {
         TagsProvider.Builder<T> builder = getBuilder();
         for (ITag.INamedTag<T> value : values) {
@@ -57,6 +58,7 @@ public interface IForgeTagBuilder<T>
         return addOptional(Arrays.asList(locations));
     }
 
+    @SuppressWarnings("deprecation")
     default TagsProvider.Builder<T> addOptional(final Collection<ResourceLocation> locations)
     {
         return getBuilder().add(ForgeHooks.makeOptionalTag(true, locations));
@@ -67,6 +69,7 @@ public interface IForgeTagBuilder<T>
         return addOptionalTag(Arrays.asList(locations));
     }
 
+    @SuppressWarnings("deprecation")
     default TagsProvider.Builder<T> addOptionalTag(final Collection<ResourceLocation> locations)
     {
         return getBuilder().add(ForgeHooks.makeOptionalTag(false, locations));

src/main/java/net/minecraftforge/common/util/ForgeNetworkTagManager.java

@@ -0,0 +1,128 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2020.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.common.util;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import javax.annotation.Nullable;
+
+import com.google.common.collect.ImmutableSet;
+
+import net.minecraft.tags.ITag;
+import net.minecraft.tags.NetworkTagCollection;
+import net.minecraft.tags.NetworkTagManager;
+import net.minecraft.tags.Tag;
+import net.minecraft.util.RegistryKey;
+import net.minecraft.util.ResourceLocation;
+import net.minecraft.util.registry.Registry;
+
+@Deprecated //For Forge's internal use only.
+public class ForgeNetworkTagManager extends NetworkTagManager
+{
+    private static Map<RegistryKey<?>, Set<ResourceLocation>> defaults = new HashMap<>();
+
+    /**
+     * Returns a new instance of this tag manager, again, only exposed for Forge's Hook use.
+     */
+    public static NetworkTagManager create()
+    {
+        return new ForgeNetworkTagManager();
+    }
+
+    /**
+     * This will provide default tags if the server side does not send us one.
+     * Note: If they DO send us a empty one, this DOES NOT override that.
+     * This is 100% intended to be only used by Forge tto make sure our tags behave correctly on vanilla servers for the edge cases where we use tags for in-game loogic.
+     * Notably: Furnace burnables.
+     */
+    public static <T> ITag.INamedTag<T> defaulted(RegistryKey<Registry<T>> type, ITag.INamedTag<T> tag, Set<ResourceLocation> values)
+    {
+        defaults.put(RegistryKey.func_240903_a_(type, tag.func_230234_a_()), ImmutableSet.copyOf(values));
+        return tag;
+    }
+
+    /**
+     * Copies the resource key from the parent tag to a new one.
+     * This simplifies the item variants of block tags.
+     * WILL explode if the parent hasn't been defaulted.
+     */
+    public static <T> ITag.INamedTag<T> defaulted(RegistryKey<Registry<T>> type, ITag.INamedTag<T> tag, RegistryKey<?> parent)
+    {
+        RegistryKey<T> regKey = RegistryKey.func_240903_a_(type, tag.func_230234_a_());
+        Set<ResourceLocation> other = defaults.get(parent);
+        if (other == null)
+            throw new IllegalStateException("Attempted to set nested default without setting parent: " + regKey + " -> " + parent);
+        defaults.put(regKey, other);
+        return tag;
+    }
+
+    private ForgeNetworkTagManager()
+    {
+        this.blocks = new ForgeNetworkTagCollection<>(Registry.BLOCK, "tags/blocks", "block");
+        this.items = new ForgeNetworkTagCollection<>(Registry.ITEM, "tags/items", "item");
+        this.fluids = new ForgeNetworkTagCollection<>(Registry.FLUID, "tags/fluids", "fluid");
+        this.entityTypes = new ForgeNetworkTagCollection<>(Registry.ENTITY_TYPE, "tags/entity_types", "entity_type");
+    }
+
+    private static class ForgeNetworkTagCollection<T> extends NetworkTagCollection<T>
+    {
+        private final ITag<T> EMPTY = Tag.func_241284_a_();
+        private final Registry<T> registry;
+        private final Map<ResourceLocation, ITag<T>> cache = new HashMap<>();
+
+        public ForgeNetworkTagCollection(Registry<T> registry, String path, String name)
+        {
+            super(registry, path, name);
+            this.registry = registry;
+        }
+
+        @Override
+        @Nullable
+        public ITag<T> get(ResourceLocation key)
+        {
+            ITag<T> ret = super.get(key);
+            if (ret == null)
+            {
+                ret = cache.get(key);
+                if (ret == null) {
+                    RegistryKey<T> regKey = RegistryKey.func_240903_a_(this.registry.getRegistryKey(), key);
+                    Set<ResourceLocation> defaults = ForgeNetworkTagManager.defaults.get(regKey);
+                    if (defaults == null)
+                    {
+                        ret = EMPTY;
+                    }
+                    else
+                    {
+                        Set<T> values = defaults.stream().map(getEntryLookup()).filter(Optional::isPresent).map(Optional::get).collect(Collectors.toSet());
+                        ret = Tag.func_241286_a_(values);
+                    }
+                    this.cache.put(key, ret);
+                }
+            }
+
+            return ret;
+        }
+    }
+
+}