I'm trying to understand a complex variance situat...

# getting-started

[CLOVIS]

I'm trying to understand a complex variance situation;

typealias NullableValue<T> = TypedValue<T, KClass<out T & Any>?>
typealias NonNullValue<T> = TypedValue<T & Any, KClass<out T & Any>>
typealias NullValue = TypedValue<Nothing?, Nothing?>

class TypedValue<T : Any?, out K : KClass<out T & Any>?>(
    val value: T,
    val type: K,
)

// Compiles, and I understand why:
val n1: NullableValue<Int> = TODO()
val n3: NonNullValue<Int> = TODO()
val n5: NullValue = TODO()

// Does not compile, and I understand why:
val n4: NonNullValue<Int?> = TODO()

However, there's this one, that I don't understand:

val n2: NullableValue<Int?> = TODO()

Type argument is not within its bounds.

Expected: Any

Found: Int?

Why does it expect Any ? From what I understand:
• NullableValue<Int?>
• is expanded to TypedValue<Int?, KClass<out Int>?>
• To me, that seems to satisfy all type bounds.
So, why is it forbidden?

[PHondogo]

Seems like compiler bug (smth with typealias) .
Such code compiles:

abstract class TypedValue<T : Any?, out K : KClass<out T & Any>?>

class TypedValue2<T> : TypedValue<T, KClass<out T & Any>?>()

val n1: TypedValue2<Int?>  = TODO()

[Daniel Pitts]

Every time I use a language with Generics, I eventually end up wishing it had templates (ala C++) instead.

[Daniel Pitts]

Using interfaces rather than typealiases, all of the lines compile:

interface TypedValue<T : Any?, out K : KClass<out T & Any>?>
interface NullableValue<T> : TypedValue<T, KClass<out T & Any>?>
interface NonNullValue<T> : TypedValue<T & Any, KClass<out T & Any>>
interface NullValue : TypedValue<Nothing?, Nothing?>

val n1: NullableValue<Int> = TODO()
val n2: NullableValue<Int?> = TODO()
val n3: NonNullValue<Int> = TODO()
val n4: NonNullValue<Int?> = TODO()
val n5: NullValue = TODO()

[Daniel Pitts]

Tangentially related, have you seen how jackson handles TypeReferences? (I think Spring has something similar). It allows you to create a reified token.

[CLOVIS]

Thanks for the link 🙂

https://youtrack.jetbrains.com/issue/KT-71462/Weird-behavior-with-typealiases-and-variance