Android Studio Build:

Android Studio 3.5 Beta 1
Build #AI-191.6707.61.35.5677133, built on June 20, 2019
JRE: 1.8.0_202-release-1483-b39-5396753 amd64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
Linux 5.0.0-25-generic

(specifically the one from the Compose repo)

Version of Gradle Plugin: 3.5.0-beta05
Version of Gradle: whatever frameworks/support/ui is using -- your setup is too damn complicated...
Version of Kotlin: 1.3.41
OS: Ubuntu 19.04

Steps to Reproduce:

Step #1: Define a @Model as a top-level class:

@Model
data class AgreementViewState(var terms: Boolean, var privacy: Boolean) {
val canProceed = terms && privacy
}

Step #2: Attempt to use that @Model in a @Composable:

@Composable
private fun Agreement() {
Padding(8.dp) {
val viewState = +model { AgreementViewState(true, false) }

Column(mainAxisAlignment = MainAxisAlignment.Start, crossAxisAlignment = CrossAxisAlignment.Start) {
Row(mainAxisAlignment = MainAxisAlignment.Start) {
Checkbox(
checked = viewState.terms,
onCheckedChange = { viewState.terms = it }
)
Text(text = "I agree to the terms and conditions")
}
Row(mainAxisAlignment = MainAxisAlignment.Start) {
Checkbox(
checked = viewState.privacy,
onCheckedChange = { viewState.privacy = it }
)
Text(text = "I agree to the privacy policy")
}
Text(text = if (viewState.canProceed) "You may proceed" else "Please indicate your agreement")
}
}
}

Expected Results: No crashes

Actual Results: A crash from generated code when the @Composable is called:

java.lang.IllegalArgumentException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkParameterIsNotNull, parameter r
at androidx.compose.frames.FramesKt._readable(Unknown Source:2)
at androidx.ui.foundation.demos.AgreementViewState.getTerms(Unknown Source:5)
at androidx.ui.foundation.demos.AgreementViewState.<init>(MainActivity.kt:230)

Note that this works:

@Model
data class AgreementViewState(var terms: Boolean, var privacy: Boolean) {
fun canProceed() = terms && privacy
}

...and this works:

@Model
data class AgreementViewState(var terms: Boolean, var privacy: Boolean) {
// fun canProceed() = terms && privacy
}

val AgreementViewState.canProceed
get() = terms && privacy

But, IMHO, the original form ought to work as well, as it seems to be the most natural form of a derived property in Kotlin.

Thanks for considering this!