Is their any way to create an extension function only available for nullable types? For example: ```...

# getting-started

[Johann Pardanaud]

Is their any way to create an extension function only available for nullable types? For example:

fun Any?.assertNotNull(): Unit = TODO()

val nullable: String? = ""
val nonNullable: String = ""

nullable.assertNotNull() // works
nonNullable.assertNotNull() // works too

The last line makes no sense because the value cannot be null, but it compiles anyway. Is it possible to improve this?

[ephemient]

String can be transparently upcast into String?, just like how all types can be safely upcast into one of their parents

[ephemient]

you could add a more specific overload and then mark it as an error,

fun Any?.assertNotNull(): Unit = TODO()

@Deprecated("Non-nullable type", level = DeprecationLevel.ERROR)
@JvmName("assertNotNull!")
fun Any.assertNotNull(): {
    // no-op
}

[Johann Pardanaud]

alright, that's what I thought, no big deal anyway but I wanted to be sure, thank you!

[Gleb Minaev]

You can also try to abuse `kotlin.internal.Exact`:

@Suppress("INVISIBLE_REFERENCE", "INVISIBLE_MEMBER")
fun <T: Any> @kotlin.internal.Exact T?.assertNotNull(): T = this ?: TODO()

fun main() {
    val nullable: String? = ""
    val nonNullable: String = ""

    nullable.assertNotNull() // works
    nonNullable.assertNotNull() // does not compile
}

[Johann Pardanaud]

Good to know! I will not use it since its internal but its really informative 😄