How can I identify when a Kotlin type will be mapped to a Java type?

Question

Context

Kotlin's kotlin.String type is currently defined as follows (1.1.2) :

public class String : Comparable<String>, CharSequence {
    companion object {}

    // Operator and override function definitions.
}

Some extensions defined on kotlin.String cast the receiving instance to the java.lang.String type to forward method invocations. For example (1.1.2) :

@kotlin.internal.InlineOnly
public inline fun String.toLowerCase(): String = (this as java.lang.String).toLowerCase()

However, nothing in the kotlin.String type definition makes it clear to me that this cast is guaranteed to succeed.

Questions

• Is there any way to easily determine if any given Kotlin type is mapped to a corresponding Java type in this manner?
• Where does this conversion actually happen? (Looking for a link to the relevant source code if possible.)

Answer 1

In Kotlin/JVM kotlin.String is one of the mapped types - a type that is represented with some existing JDK class in runtime.

Therefore a cast between kotlin.String and java.lang.String succeeds in runtime, even though these are two unrelated types from the point of Kotlin type system.