Facing a weird case here, I have the following fun...

# announcements

[robin]

Facing a weird case here, I have the following function:

fun <T> test(param: T) {
    val clazz = param::class
}

And the compiler complains (rightly so) that param has a nullable type 'T'. It even suggests using !! to make it non-nullable, but if I do that, the compiler still complains with the exact same message. Here are a few more variations I've tried, each one with the same error message:

val clazz = param!!::class
val clazz = param?.let { it::class }
val clazz = if (param != null) param::class else null

So, how am I supposed to get to the class of param here? Is this a bug with type inference or am I missing something? Making T reified or constrain it to Any is not an option.

[robin]

Okay, safe-casting to Any inside the function works:

val clazz = (param as? Any)?.let { it::class }

But that seems... Unneccessarily convoluted.

[Shawn]

why can’t you constrain it to Any?

[robin]

Because the function I'm actually doing this in is overriding a function from an interface I have no control over.

[karelpeeters]

I definitely think this is a bug: extensions functions do in fact work fine:

fun Any.foo() = print(this.toString())

fun <T> bar(param: T) {
    if (param != null) {
        println(param.foo())
        println(param::class)
    }
}

[karelpeeters]

Only the :class line has an error here. Changing the parameter type to Any? also fixes the issue. Can you report this on YouTrack?

[robin]

Yes, will do!

[karelpeeters]

Looks like someone got there before you did: https://youtrack.jetbrains.net/issue/KT-20778

[robin]

Ah right, thank you! And it's already 11 months old...

[robin]

Added my workaround there for completeness.

[karelpeeters]

(x as Any?)?.let { it::class } works too and doesn't do the useless INSTANCEOF java/lang/Object check at runtime simple smile

[robin]

Yeah that's better, didn't even think about that 😄

[Dico]

Um, I have no clue what param!!::class does.

Aren't you looking for param.javaClass? Just requires it to be non-null. Well, just add a constraint <T : Any>

[karelpeeters]

As discussed that's not an option, and this is definitely a bug.

[karelpeeters]

Casting T to Any? shouldn't affect whether null checks work, see my minimal code example.

[robin]

Yup, not an option, although I think param?.javaClass might work in my case, but won't work if you actually need the KClass.

[Dico]

Yeah, I agree that what I just tested is behaving unlike what I would expect.

[robin]

param!!::class is just asserting param to be non-null, and then getting it's KClass. At least that's what it should do.

[Dico]

Oh right, ::class can give you the KClass of the object

[Dico]

You can try ?.javaClass?.kotlin for now

[Dico]

But yeah, weird thing is that a non-null assertion of a generic type parameter with default upper bound Any? is not considered to be a subtype of Any

[karelpeeters]

It is, see my code 😛. It just doesn't work for ::class for some reason.

[Dico]

Alright, I guess ::class is a special case then.

[karelpeeters]

Must be, I'm looking around in the source.

[karelpeeters]

https://github.com/JetBrains/kotlin/blob/d7f2eeb6e80565fdbf8cd125bb744ec0d6aabb2c/compiler/frontend/src/org/jetbrains/kotlin/types/expressions/DoubleColonExpressionResolver.kt#L133

[karelpeeters]

Maybe that's called before smart casting etc is done? I don't have a debugging environment for the compiler set up unfortunately.