Why is the compiler not able to smart cast the mem...

# getting-started

[hfhbd]

Why is the compiler not able to smart cast the member s to Int in this case: is Foo.A, is Foo.B

public sealed interface Foo {
    public val s: Int?
    
    public data class A(override val s: Int): Foo
    public data class B(override val s: Int): Foo
    public data class C(override val s: Int?): Foo
}

public fun a(foo: Foo) {
    when (foo){
        is Foo.A -> foo.s + foo.s // smart cast to A and s is Int
        is Foo.B -> foo.s + foo.s // smart cast to B and s is Int
        is Foo.C -> (foo.s ?: 0) + (foo.s ?: 0) // smart cast to C, but s is Int?
    }
    when (foo){
        is Foo.A, is Foo.B -> foo.s + foo.s // no smart cast, Foo.s is Int?
        is Foo.C -> (foo.s ?: 0) + (foo.s ?: 0)
    }
}

[ephemient]

the two overrides are not related to each other. you're expecting structural typing but there's only nominal typing

[ephemient]

you could force them to be related with

sealed interface Foo2 {
    override val s: Int
}
data class A(override val s: Int) : Foo2
data class B(override val s: Int) : Foo2

[hfhbd]

Yeah, that’s my workaround

[hfhbd]

But in this case, the compiler inferred the type as `Comparable<*> & Serializable`: val s = if (true) "" else 42. So it could also be inferred.

[ephemient]

that's different. those are two separate codepaths being type-unified, whereas you have a single conditional branch

[ephemient]

the compiler does not duplicate the code for each disjunction in your condition