Hello - I am upgrading a project from Kotlin 1.3.72 to 1.4.21. As part of this, I had some code of t...

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[tpgillam]

Hello - I am upgrading a project from Kotlin 1.3.72 to 1.4.21. As part of this, I had some code of the form val x = foo() ?: return moo, where foo(): T? is a method returning a nullable generic T?. Now previously, x was of type T, however now the IDE is telling me that it is of type T!!. I can seemingly force it to be of type T, however can somebody explain what is going on here? I also can't find the relevant part of documentation that explains what !! means when postfixing a (possibly only generic?) type. Thanks!

[Marc Knaup]

Can you share a screenshot?
A type should never be inferred to be Something!!. That’s probably either an IDE bug or a compiler bug.

[tpgillam]

This extra level of indirection seems to be required to actually get a compile error:

[tpgillam]

image.png

[tpgillam]

(The pop-up window isn't getting screenshotted for some reason!)

[tpgillam]

But it is happy with this:

[tpgillam]

image.png

[Marc Knaup]

Can you share as text so we can play with it? 🙂

You should enable type hints so that you see the type directly in IDE editor.
Preferences > Editor > Inlay Hints > Kotlin > Types > Show hints for > Local variable types

[tpgillam]

data class Wrapper<T>(val value: T)

class Moo<T> {
    fun moo(): T? {
        return null
    }

    fun foo(): List<Wrapper<T>> {
        val x = moo() ?: return listOf()
        val list = mutableListOf<Wrapper<T>>()
        list.add(Wrapper(x))
        return list
    }
}

[Marc Knaup]

Yeah looks like a compiler bug.

[Marc Knaup]

Minimal repro:

[tpgillam]

(I'm generally happy just using ctrl-shift-P personally 🙂 but yep:

[tpgillam]

image.png

[Marc Knaup]

wait

[Marc Knaup]

maybe it’s even correct

[Marc Knaup]

hmm. It’s actually good

[Marc Knaup]

T is nullable. So T!! literally means a non-null T.

[tpgillam]

Ah- yep (makes sense after your edit)

[tpgillam]

Right - so I need to do something like this to ensure that T actually means a type that isn't null: class Moo<T: Any> . Then the compiler is happy that Wrapped<T!!> is the same as Wrapped<T>, I think.

[tpgillam]

Thanks very much!

[Marc Knaup]

Thank you too 🙂