Is there a way to tell the Kotlin compiler to treat certain Java methods with a

void

return as

Nothing

instead of

Unit

?

That would break the type system.

void

is isomorphic to

Unit

- they are both sets with exactly one element - whereas

Nothing

is a set with zero elements. There would be no way for Java to interoperate with Kotlin if

Nothing

was used in place of

void

.

I only meant one way, so in my Kotlin code I tell it "this function from this library never returns". I don't need Java to know anything about it.

The type checker needs to be able to understand it and they are fundamentally different types

Yes, but I don't see how that's an issue. You can already use things like annotations (even external annotations) to modify compilation. Why not to say "this is a divergent code path"?

Eh, my guess is writing something to handle that special case of Java not really having an official Nothing type wasn't a high priority for the Kotlin roadmap

That's fair

Strangely enough it's possible to express what you want in Kotlin with enough function wrapping, although IJ warns that the call is "unreachable code"

It doesn't take all that much.

inline fun diverge(op: () -> Unit): Nothing {
    op()
    error("Cannot be reached")
}

That seems like an easier solution than adding exceptions to the type system

I didn't need it in the type system. Just something that would work like external annotations and would interpreted by the compiler.