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abstract class Base {
    @Composable
    abstract fun run(value: Modifier = Modifier)
}

class Real: Base() {
    @Composable
    override fun run(value: Modifier) {
        println(value)
    }
}

fun main() {
    Window {
        Real().run()
    }
}

Running this gives Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Parameter specified as non-null is null: method Real.run, parameter value. Removing the @Composable annotation from the two methods doesn't crash.

And Kotlin doesn't let you specify the default value "again" in the Real.run function.