Why does `as? TArgument` give an `Unchecked cast` ...

# announcements

[Kroppeb]

Why does as? TArgument give an Unchecked cast warning

[Shawn]

this question needs some more context in order to be answered precisely

[Kroppeb]

This is the function

fun <TArgument:Any> ArgumentParser<TArgument>.value(): TArgument?{
	return arguments[this] as? TArgument
}

[Kroppeb]

I thought as? was a safe cast?

[karelpeeters]

You can't actually safe cast to generics on the JVM because of erasure.

[karelpeeters]

That's what "unchecked cast" means: https://stackoverflow.com/a/262539/5517612

[streetsofboston]

But as? should safely return null if the cast fails

[Shawn]

that’s not what that means

[streetsofboston]

https://kotlinlang.org/docs/reference/typecasts.html

To avoid an exception being thrown, one can use a safe cast operator as? that returns null on failure

[karelpeeters]

That function gets compiled to:

fun ArgumentParser<Any?>.value(): Any?{
    return arguments[this]
}

[karelpeeters]

That's what erasure means. How could it possibly check anything about TArgument? It doesn't even know about it.

[streetsofboston]

Ha!!!! I read TArgument as a type, not as a generic type.
Thanks!

[streetsofboston]

@Kroppeb The warning is not about the as? but about the return-type vs the type that is actually returned. 🙂

[Kroppeb]

?

[Kroppeb]

It's only marking that piece of code

[Kroppeb]

the as? TArgument

[Kroppeb]

The thing is that I know that the value will be TArgument if the key is in the dict, but there is no way to tell this to the compiler, right? The dictionary maps ArgumentParser<T> -> T

[streetsofboston]

You’ll have to remove the type-erasure to be sure the return type matches the value returned from arguments[this]. Make the type reified:

inline fun <reified TArgument:Any> ArgumentParser<TArgument>.value(): TArgument? { ... }

[karelpeeters]

Yes, your code could very well be correct, the warning is saying that you have to make very sure that you're right, because not only is compile-time safety broken (which is always the case for casts) but even the runtime can't check your cast because it's a generic type! This can introduce very hard to catch bugs.

[Kroppeb]

But then arguments has to be public no?

[streetsofboston]

yup.

[karelpeeters]

Or use @PusblishedApi.

[Kroppeb]

?

[streetsofboston]

@PublishedApi doesn’t work on private, only on internal scope…

[karelpeeters]

But if you're sure this cast is correct feel free to suppress it, that's quite common when writing your own datastructures. There's no real need for reified.

[Dico]

There is a bug report about this.
as? On a type parameter is the same as as on a type parameter.

[karelpeeters]

(it still checks the bound so it's not exactly the same (I know I'm being pedantic))

[Dico]

Wait, what's the difference?

[karelpeeters]

For example:

fun <T: Any> foo() {
    null as? T  //succeeds
    null as T   //fails
}