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Another one with nullability

Let’s assume I have a function `fun doSomething(input: SomeClass&lt;Concrete?&gt;)`.

Use case is:
```
val localInstance: SomeClass&lt;Concrete&gt; = ...
doSomething(localInstance)
```

Should I naively cast local instance to `localInstance as SomeClass&lt;Concrete?&gt;` to use this method? Assume that I can’t change the method.

Can you change `SomeClass`? It works if you make its type parameter covariant. `class SomeClass&lt;out T&gt;`

This makes `SomeClass&lt;Concrete&gt;` a subtype of `SomeClass&lt;Concrete?&gt;`.

Then this method call cannot be done. The method (`doSomething`) might be trying to put a `null` into your `SomeClass`. If you were to pass in a `SomeClass&lt;Concrete&gt;` that would cause problems.

You need to change either the class or the method.

And what if I’d be able to change the method?

I think you can do this: ```fun doSomething(input: SomeClass&lt;out Concrete?&gt;)```