Hi👋, Can someone tell me why the compile error is going away as soon as uncomment the

student as Student

from the below code? Don’t I have to explicitly assign the cast result to a variable to access the

score

property?

abstract class Person(val name: String)
class Student(val score: Int, name: String) : Person(name)
class Teacher(name: String) : Person(name)

fun someFun(student: Person) {
//  student as Student
    
    println(student.score)
}

Kotlin has flow-sensitive typing, so it can smart cast the

student

variable to a more precise type (

Student

) after the

as

cast even without using a separate variable (simply because it knows the cast would have failed otherwise). It's the same principle as null checks that smart cast to non-null type inside the

if

.

I’d imagine it will work if I add the below statement instead of the

student as Student

. But it is not working

assert(student is Student)

I guess

assert

doesn't have a contract, or maybe not one that's precise enough to help the compiler here (EDIT: this is actually because

assert

uses JVM assertions, which can be disabled). OTOH, the cast with

as

operator is pretty obvious to the compiler, and it's reliable.

Even

check(student is Student)

is working

Yes, that works too. But keep in mind that they don't have the same semantics.

check

and

require

don't throw the same exception (

IllegalStateException

vs

IllegalArgumentException

respectively). You should use

require

if it's input that needs to be checked, and

check

if you want to verify that some state matches your expectations at some point (e.g. initialization has happened before usage, things like that).