Is it somehow possible to have smart casts with ty...

# announcements

[diesieben07]

Is it somehow possible to have smart casts with type parameters? I have a situation where I have
interface Base<T> and Concrete<T : Any> : Base<T>. In a method which accepts Base<T> I now need to check is Concrete<T>, but that's impossible because in the method T is not constrained to Any (but it has to be, if I have a Concrete<T>...
See thread for more code.

[diesieben07]

interface Base<T> {
    fun foo(): T = TODO()
}
class Concrete<T : Any> : Base<T> {
    fun otherFoo(): T = TODO()
}

fun <T> acceptBase(base: Base<T>): T = when (base) {
    is Concrete<*> -> acceptConcrete(base) // <- this does not compile
    else -> base.foo()
}

fun <T : Any> acceptConcrete(c: Concrete<T>) = c.otherFoo()

[Chris Ruddell]

try this:

fun <T: Any> acceptBase(base: Base<T>): T = when (base) {
        is Concrete -> acceptConcrete(base)
        else -> base.foo()
    }

[diesieben07]

Yes, but I can't call this with a Base<Foo?>

[Chris Ruddell]

If you need it to be optional, try this:

interface Base<T> {
        fun foo(): T = TODO()
    }
    class Concrete<T : Any?> : Base<T> {
        fun otherFoo(): T = TODO()
    }
    fun <T: Any?> acceptBase(base: Base<T>): T = when (base) {
        is Concrete -> acceptConcrete(base) // <- this does not compile
        else -> base.foo()
    }
    fun <T : Any?> acceptConcrete(c: Concrete<T>) = c.otherFoo()

    class Foo {

    }

    fun test() {
        val base = acceptBase(Concrete<Foo?>())
    }

[diesieben07]

This allows me to make a Concrete<Foo?>, which must be forbidden. Concrete refines the constraints on Base. Base allows nulls, Concrete does not.

[Scott Whitman]

does inline / reified work?

interface Base<T: Any> {
    fun foo(): T = TODO()
}
class Concrete<T : Any> : Base<T> {
    fun otherFoo(): T = TODO()
}
inline fun <reified T: Any> acceptBase(base: Base<T>): T = when (base) {
    is Concrete -> acceptConcrete(base) // <- this does not compile
    else -> base.foo()
}
fun <T : Any> acceptConcrete(c: Concrete<T>): T = c.otherFoo()

[diesieben07]

I cannot call acceptBase with a Base<Foo?> there.

[diesieben07]

And you have also changed Base to no longer accept nulls.