CSSStyleVariable APIs allow setting values of the wrong type

[bitspittle]

Simple example code to demonstrate the problem (here, setting a LineStyle variable with a DisplayStyle value):

val unexpectedVariableAssignment by variable<LineStyle>()

@Composable
fun demoUnexpectedVariableAssignment() {
    Div(attrs = {
        style { 
            unexpectedVariableAssignment(DisplayStyle.Flex)
        }
    })
}

Expected: The line unexpectedVariableAssignment(DisplayStyle.Flex) should not compile

Actual: The code compiles and runs fine.

[bitspittle]

Digging a bit more, it seems this is because CSSStyleVariable's class declaration uses out variance:

class CSSStyleVariable<out TValue : StylePropertyValue>(override val name: String) : CSSVariable

and the call for invoking a variable has the following API:

operator fun <TValue : StylePropertyValue> CSSStyleVariable<TValue>.invoke(value: TValue) { ... }

so it seems like what is happening is the compiler sees the line unexpectedVariableAssignment(DisplayStyle.Flex) and sees that LineStyle and DisplayStyle are both StylePropertyEnums, so it chooses StylePropertyEnum as the generic type to satisfy invoking the variable.

[bitspittle]

Now I'm wondering why CSSStyleVariable even uses covariance?

What would happen if you took the out out?

class CSSStyleVariable<T : StylePropertyValue>(override val name: String) : CSSVariable

[okushnikov]

Please check the following ticket on YouTrack for follow-ups to this issue. GitHub issues will be closed in the coming weeks.