Reading 2nd edition of Kotlin in Action.

There is an example with an unsafe cast to a generic List<T> type with an Int assumption for type argument:

fun printSum(c: Collection<*>) {
    val intList = c as? List<Int> // If 'c' is a `List`, Int is not guaranteed because of type erasure
        ?: throw IllegalArgumentException("List is expected")
    println(intList.sum())
}

The code compiles with a WARNing on unsafe cast and passing a list of strings:

printSum(listOf("a", "b", "c"))

will explode with java.lang.ClassCastException:

Exception in thread “main” java.lang.ClassCastException: class java.lang.String cannot be cast to class java.lang.Number (java.lang.String and java.lang.Number are in module java.base of loader ‘bootstrap’)

The ClassCastException is expected, but why the error message complains on a java.lang.Number and not on an Int/java.lang.Integer?

There is not direct references to java.lang.Number class in the code and the sum() function called is an extension on Iterable<Int> (not Iterable<Number>).