Hi again, can anybody help me with kotlin multiplatform and k2 problem? I am encountering the follo...

# multiplatform

[Michal Lepíček]

Hi again,
can anybody help me with kotlin multiplatform and k2 problem?

I am encountering the following error:

The following declaration is incompatible because return type is different:
    val name: String?

In module A, I have an interface with:

abstract val name: String?

In module B, there is a child class with

override val name: String = "SampleDestination"

Additionally, both modules contain actual classes with empty expect classes. This setup prevents successful compilation on both iOS and Android platforms.

This is a simple example, which is easily avoidable. However, there are more complicated cases that fail with the same error.
This code works on k1.
It also works if I keep all of it in one multiplatform module which is why I believe it to be a bug.

[streetsofboston]

You tighten the type of name (from String? to String) in the base class B, which shouldn't be allowed. I think K2 is correct

[streetsofboston]

Eg if you pass an instance of B around that is declared as of type A, and you assign null to name you'd have a problem: type A allows it but type B can't handle it.

[Michal Lepíček]

That doesn't seem right, since it works if it is in the same module, or does it?

[Michal Lepíček]

And as I have written, it is just the simplest reproducible. I still need to change the type from A to B, where B inherits from A.
Same error and pretty sure a valid case.

[streetsofboston]

Nothing to do with modules. It's about subtyping, assignability.

[streetsofboston]

a.name is nullable. The compiler thinks it's alright to assign null to it.
However, you create an instance of B, where b.name is not nullable.
If you do val a: A = B() and then a.name = null, you'll have a problem

[Michal Lepíček]

I think your counter comments are only valid if the code was mutable, but it's all immutable.
And returning to my comment, we can forget about (non)optional. Same error comes with immutable non optional A, B classes where B inherits from A.

[Michal Lepíček]

"Nothing to do with modules". Well, it does. Since it works in one module, but breaks in multiple.

[streetsofboston]

Ah, yes, you're right... It's val, not var.

[streetsofboston]

Sorry... Then it's indeed an issue, that K2 can't figure this out when it crosses a module boundary.

[Michal Lepíček]

No worries, thanks for confirming it to be an issue. I have copy pasted it to youtrack with sample project
https://youtrack.jetbrains.com/issue/KT-68980/KMP-2-incompatible-because-return-type-is-different