In 1.12.2, it was possible for Iron Golems to spawn within half slabs, as long as the block they stood on was a solid, opaque block. This allowed for very efficient spawning platforms in advanced iron farms, utilising many slabs on top of each other so that the maximum amount of spawning spaces for iron golems could be achieved.
However, in 18w30b (and possibly the initial 1.13 release) Iron Golems need to have completely cleared space above the block they stand on to be spawned. This greatly reduces the efficiency of these spawning platforms that are the core of many large iron farms.
Though it is bad to allow golems to spawn within full blocks (as was the case in a recent bug), they should still be able to spawn within half slabs, like they have been in 1.12 and previous versions.
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Hmm. Unfortunately then it seems that either way, some people will be upset, but now I don't know which way it was originally meant to be! But I can say with certainty that golems don't spawn in slabs as of 18w30b (I tested this using one of those spawn pads i mentioned), so MC-3927 should be marked as resolved now.
Anyway, I believe that it would have been better to keep golem spawning the way it was in 1.12, when they could spawn in slabs, so that existing farms can continue to work in 1.13 as they have been in previous versions.
I can confirm this. Just tested using the Iron Casster, and the golems only spawn on the top surfaces of the spawn pads, not within them like they should be (or at least used to be) able to.
I have a feeling this will be marked as Works as Intended, because although it is bad for the iron farms 😞, no other mobs spawn in slabs.Â
See MC-3927.
Which one is it? They can spawn in slabs or not?