Why does this give a `ClassCastException` (for the...

# compose

[Mark]

Why does this give a ClassCastException (for the first of the Pair):

internal fun Pair<@Composable () -> String, (() -> Unit)?>.toButton(): @Composable () -> Unit = {
    var (label, onClick) = this@toButton // same runtime exception if use `this.first` instead
    ...
}

[shikasd]

Can you paste the exception? Also looks like a bug, feel free to file an issue on tracker.

[Mark]

I believe I’ve discovered the issue. I have a property declared like:
var positiveButton: (Pair<@Composable () -> String, T>)? = null

Usually, I assign it like:
positiveButton = @Composable { stringResource(id = labelResId) } to value

But sometimes like:
positiveButton = { label } to value
and that is what is causing the crash.

Instead, I just need to assign like:
positiveButton = @Composable { label } to value

Surely this is a bug? Either the compiler should pick it up, or the runtime should allow it.

[shikasd]

Yes, that's definitely a bug

[Mark]

Can you reproduce it?

[shikasd]

Didn't try to yet, but feel free to file a bug on the tracker :)

[Mark]

Perhaps same as this one: https://youtrack.jetbrains.com/issue/KTIJ-23723/False-positive-TYPEMISMATCH-with-Composable-lambda-as-a-type-and-the-second-item-in-List

[shikasd]

Just to leave a note here, I investigated this issue and seems like it is a limitation of compiler plugin in K1. It doesn't propagate the type of the receiver to the compiler plugin and in this case { label } is a receiver for the to call. From what I see, it should be properly fixed in K2 inference, but cannot predict if that's the case atm.