Carpets can be placed on top of light blocks

Can confirm in 21w17a.

Why this isn't resolved as a duplicate of MC-17833?

@yusufulus, because carpets cannot be placed on air. 

Okay

Relates to MC-17833

Can confirm in 1.17.

Can confirm in 1.17.1.

I am able to confirm this behavior in 21w43a. Here are some extra details regarding this problem.

The Bug:

Carpets can be placed on top of light blocks.

Steps to Reproduce:

• Build the setup as shown in the attachment below.

• Obtain a light block and a carpet.

/give @s minecraft:light
/give @s minecraft:white_carpet

• Place down the light block on top of the diamond block.

• Place down the carpet on top of the light block by looking at the gold block.

• Take note as to whether or not carpets can be placed on top of light blocks.

Observed Behavior:

Carpets can be placed on top of light blocks.

Expected Behavior:

Carpets would not be able to be placed on top of light blocks.

Can confirm in 1.18.

Can confirm in 1.18.1.

Here's a code analysis of this issue.

Code Analysis:

The following is based on a decompiled version of Minecraft 1.18.1 using MCP-Reborn.

net.minecraft.world.level.block.CarpetBlock.java

public class CarpetBlock extends Block {
   ...
   public boolean canSurvive(BlockState $bs, LevelReader $lr, BlockPos $bp) {
      return !$lr.isEmptyBlock($bp.below());
   }
}

net.minecraft.world.level.LevelReader.java

public interface LevelReader extends BlockAndTintGetter, CollisionGetter, BiomeManager.NoiseBiomeSource {
   ...
   default boolean isEmptyBlock(BlockPos $bp) {
      return this.getBlockState($bp).isAir();
   }
   ...

If we look at the above classes, we can see that in order for a carpet block to survive, the block directly below it must not be air. If it is air, the carpet cannot be placed/will be destroyed. Since a light block isn't air, carpets can be supported by them, resulting in this problem occurring.