@bjmi Thanks for creating the issue! I think this would be a natural addition. Would you be interested in submitting a PR?

I can give it a try.

@mikhail2048 / @bjmi there are two PRs for this now.

It would be great for this implementation to use Kotlin's contracts feature so the instance can be used subsequently as an instance of the expected class.

See also: https://youtrack.jetbrains.com/issue/KT-28298 which has been implemented.

A slight modification of #3492:

inline fun <reified T : Any> assertInstanceOf(actualValue: Any?, message: String? = null): T {
    contract {
        returns() implies (value is T)
    }
    return Assertions.assertInstanceOf(T::class.java, actualValue, message)
}

will allow this pattern without cast:

val bar: Any = foo()
assertInstanceOf<String>(bar)
bar.lowercase() // bar is a String

@andrewparmet you can leave a suggestion as a review on a PR in context, more likely to get noticed when the PR is reviewed.