In 18w50a, Villagers no longer have trades by default after specifying NBT data. This was not the case in prior snapshots, when villagers had the Profession tag.

How to reproduce:

1. Summon a villager with VillagerData set to

   {profession:"minecraft:toolsmith",level:5,type:"minecraft:taiga"}

   Seems like a useful villager, right?
            →  Right click on it, only to be dismayed that the high-ranking villager has absolutely nothing to trade.