why I’m able to pass `(Something)->R?` instead ...

# stdlib

[miha-x64]

why I’m able to pass (Something)->R? instead of (Something)->R?

private inline fun <reified K : Enum<K>, V, R> EnumMap<out K, V>.mapValuesIfNotNull(transform: (Map.Entry<K, V>) -> R?): Map<K, R> {
    return mapValuesTo(EnumMap<K, R>(K::class.java), transform) //                             nullable return type ^
}