The bug
In versions before 17w47a you could use block_id:meta_data to get non-default block states for superflat presets. Due to the internal changes which happened in 17w47a meta data does not exist anymore, however there was not added another way to specify block states. Trying to use square brackets crashes the game (see MC-122441).
How to reproduce
Try to use the following superflat preset
minecraft:oak_slab[type=top];127;decorationLinked issues
Comments 8
confirmed still in 18w22c
Also: If you modify the level.dat to force the string minecraft:oak_slab[type=top];127;decoration to be the generator options, the world simply refuses to load
Confirmed for 1.13-pre5
Cannot confirm for 1.13-pre6, as this issue is currently blocked by MC-132631.
Confirmed for 19w04b and 1.13.2
Confirmed for 1.14.1
Duplicate of MC-118624
Still affects 18w05a, please update!