example.kt

open class OpenClass
class FirstOpenClass : OpenClass()
class SecondOpenClass : OpenClass()

class Foo<TGen : OpenClass>(
    val open: TGen = FirstOpenClass() as TGen
) {
    companion object {
        fun build() = Foo(FirstOpenClass()) //it’s OK
    }
}

val a: SecondOpenClass = Foo<SecondOpenClass>().open