rspec/rules/S6202/kotlin/rule.adoc

== Why is this an issue?

The `is` construction is a preferred way to check whether a variable can be cast to some type statically because a compile-time error will occur in case of incompatible types. The `isInstance()` functions from https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.reflect/-k-class/is-instance.html[`kotlin.reflect.KClass`] and https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html#isInstance-java.lang.Object-[`java.lang.Class`] work differently and type check at runtime only. Incompatible types will therefore not be detected as early during development, potentially resulting in dead code. `isInstance()` function calls should only be used in dynamic cases when the `is` operator can't be used.


This rule raises an issue when `isInstance()` is used and could be replaced with an `is` check.


=== Noncompliant code example

[source,kotlin]
----
fun f(o: Any): Int {
    if (String::class.isInstance(o)) {  // Noncompliant
        return 42
    }
    return 0
}

fun f(n: Number): Int {
    if (String::class.isInstance(n)) {  // Noncompliant
        return 42
    }
    return 0
}
----


=== Compliant solution

[source,kotlin]
----
fun f(o: Any): Int {
    if (o is String) {  // Compliant
        return 42
    }
    return 0
}

fun f(n: Number): Int {
    if (n is String) {  // Compile-time error
        return 42
    }
    return 0
}

fun f(o: Any, c: String): Boolean {
    return Class.forName(c).isInstance(o) // Compliant, can't use "is" operator here
}
----