filterNotNull on Kotlin List with generic type

Question

This is working :

val values: List<String>  = listOf("a", null, "b").filterNotNull()

This is not working :

fun <A> nonNullValues(values: List<A?>): List<A> = values.filterNotNull()

The compiler complains about generic types :

Error:(8, 63) Kotlin: Type parameter bound for T in fun <T : kotlin.Any> kotlin.collections.Iterable<T?>.filterNotNull(): kotlin.collections.List<T>
 is not satisfied: inferred type A is not a subtype of kotlin.Any

This one is working :

fun <A: Any> nonNullValues(values: List<A?>): List<A> = values.filterNotNull()

Could someone please explain me why I need to tell the compiler that A is a subtype of Any? I was thinking every type was a subtype of Any...

Thanks!

Answer 1

As per the Kotlin docs:

The default upper bound (if none specified) is Any?

This means that the problematic declaration is equivalent to:

fun <A:Any?> nonNullValues(values: List<A?>): List<A> = values.filterNotNull()

The nonNullValues method declares to return a list of nullable items of type A whereas filterNotNull returns a list of non nullable items of type A. Hence a mismatch and a compiler error.