Why does List<Int> is MutableList<Int> evaluate to true in Kotlin?

In the following Kotlin code snippet:

fun main() {
    val list = listOf(1, 2, 3)
    println(list is MutableList<Int>) // true
}

I expect that list of type List<Int> should not be considered a MutableList<Int>, but the compiler outputs:

Condition 'list is MutableList' is always true.

I understand that MutableList is a subtype of List. Is this some form of downcasting? How exactly does the is operator work in this context?

I've read the documentation on the is operator, but I still can't find an explanation for this specific behavior:

• Kotlin Keyword Reference
• Type Casts in Kotlin

I'm also interested in diving into the source code of the is operator, but it's hard for me to find it in https://github.com/JetBrains/kotlin.

Kotlin Version: 2.0.10

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Kotlin uses Java collection classes, which only have a single List interface containing both read and mutation methods. Compile-time references to List and MutableList are both compiled to java.util.List references. You can see the detailed answer here, 1 user asked a similar question to yours: https://stackoverflow.com/a/52591274/20536350