For reference: question about this on SO

Besides fixing the obvious mistake it would also be great to clarify the behaviour regardin union types.

It's not true that an alphabetical order is used. First the primitive ones are used https://github.com/crystal-lang/crystal/blob/master/src/json/from_json.cr#L192
and the the alphabetical of non primitives is used (but mainly as a side effect of how unions are stored)

@bcardiff I'm half tempted to use the dreaded words "implementation-defined behaviour" to describe the order to make it open to optimization past 1.0...

I would say "implementation defined". If two models can be parsed without problem from a same JSON, then you can't rely on which one you'll get and you probably have to use some other way to parse that.

Changed to just "undefined". It doesn't really matter to the user if it's implementation-defined or not. It's simply behaviour they cannot rely on, hence undefined.