kotlinObservables.kt

interface KotlinConsumer<T> {
    fun accept(value: T)
}
class KotlinObservable<T> {
    fun subscribe(onNext: KotlinConsumer<T>) {}
}

class Test {
    fun main() {
        // FAILS: Type Mismatch: inferred type is KFunction1<@ParameterName String, Unit> but KotlinConsumer<String> was expected
        KotlinObservable<String>().subscribe(this::stringConsumer)

        // FAILS:  Type mismatch: inferred type is () -> ??? but KotlinConsumer<String> was expected
        KotlinObservable<String>().subscribe({ println(it) })

        // Works: Anonymous Implementation of the class
        KotlinObservable<String>().subscribe(object: KotlinConsumer<String> {
            override fun accept(value: String) { println(value) }
        })
    }

    // I want to use this method, but can't
    fun stringConsumer(value: String) {
        println(value)
    }
}