interface Foo<T> {
    fun getValue(): T
    fun removeValue()
    fun setValue(value: T) // is there a way to only allow non-null values?
}

interface NonNullFoo<T: Any>: Foo<T>
interface NullableFoo<T: Any>: Foo<T?>

fun setValue(value: T & Any)

val <T> Foo<T>.valueProperty: ReadWriteProperty<Any, T>
    get() = object : ReadWriteProperty<Any, T> {
        override fun getValue(thisRef: Any, property: KProperty<*>): T {
            return getValue()
        }

        override fun setValue(thisRef: Any, property: KProperty<*>, value: T) {
            setValue(value) // compile error: require T & Any
        }
    }