Is there a contract to let Kotlin know that my member `val`'s value will stay the same? Basically I need smart casting to work on

thing

in

data class Wrapper(val thing: String?)

(from another module of course).

It will work as long as the property doesn't have a custom getter, or is delegated.

From another module? I don't think so.

Why would it work differently from another module @diesieben07?

Because of separate compilation. Someone might compile something with your module and your property is in a data-class. You then switch that property to be a computed one (custom getter), which means the smart-cast would no longer be allowed, but the other module is already compiled with the smart-cast in place.

Seems a bit weird to me. It wouldn't be any different than, let's say, removing a function, right? If the other module was already compiled with a call to that function, it would throw a

NoSuchMethodError

. In this case, I think it would throw an NPE because of the null-check the compiler inserts when it smart casts.

So, changing the property to a custom getter or delegate would just be a breaking change.

Yes, this is a tradeoff. The Kotlin creators chose to make this not a breaking change

Right 👍