package com.azizd.thunderbird.enchant;

import com.google.gson.JsonObject;
import net.minecraft.enchantment.Enchantment;
import net.minecraft.enchantment.EnchantmentType;
import net.minecraft.enchantment.Enchantments;
import net.minecraft.inventory.EquipmentSlotType;
import net.minecraft.inventory.Inventory;
import net.minecraft.item.ItemStack;
import net.minecraft.item.crafting.FurnaceRecipe;
import net.minecraft.item.crafting.IRecipeType;
import net.minecraft.loot.LootContext;
import net.minecraft.loot.conditions.ILootCondition;
import net.minecraft.util.ResourceLocation;
import net.minecraftforge.common.loot.GlobalLootModifierSerializer;
import net.minecraftforge.common.loot.LootModifier;
import net.minecraftforge.items.ItemHandlerHelper;

import javax.annotation.Nonnull;
import java.util.ArrayList;
import java.util.List;

public class EnchantAutoSmelt extends Enchantment{
    public EnchantAutoSmelt(Rarity rarityIn, EnchantmentType typeIn, EquipmentSlotType... slots) {
        super(rarityIn, typeIn, slots);
    }

    @Override
    public int getMaxLevel() { return 1; }

    @Override
    protected boolean canApplyTogether(Enchantment ench) {
        return ench != Enchantments.SILK_TOUCH && ench != Enchantments.FORTUNE && super.canApplyTogether(ench);
    }
    private static class EnchantAutoSmeltLoot extends LootModifier{

        public EnchantAutoSmeltLoot(ILootCondition[] conditionsIn) {
            super(conditionsIn);
        }

        @Nonnull
        @Override
        public List<ItemStack> doApply(List<ItemStack> generatedLoot, LootContext context) {
            ArrayList<ItemStack> ret = new ArrayList<ItemStack>();
            generatedLoot.forEach((stack) -> ret.add(smelt(stack, context)));
            return ret;
        }

        private static ItemStack smelt(ItemStack stack, LootContext context) {
            return context.getWorld().getRecipeManager().getRecipe(IRecipeType.SMELTING, new Inventory(stack), context.getWorld())
                    .map(FurnaceRecipe::getRecipeOutput)
                    .filter(itemStack -> !itemStack.isEmpty())
                    .map(itemStack -> ItemHandlerHelper.copyStackWithSize(itemStack, stack.getCount() * itemStack.getCount()))
                    .orElse(stack);
        }

        private static class Serializer extends GlobalLootModifierSerializer<EnchantAutoSmeltLoot> {
            @Override
            public EnchantAutoSmeltLoot read(ResourceLocation name, JsonObject json, ILootCondition[] conditionsIn) {
                return new EnchantAutoSmeltLoot(conditionsIn);
            }

            @Override
            public JsonObject write(EnchantAutoSmeltLoot instance) {
                return null;
            }
        }

    }
}

This is my code. Everything works but the smelting part. The drops are not smelted. 

You need to register the serializer and add the json data as well (including adding your serializer to the forge loot_modifier.json).